May one kill a mosquito on Shabbos?

Last week, I received the following by email, from the New York City Dept of Health and Mental Hygiene:

West Nile Virus Detected in New York City Mosquitoes Health Department reminds New Yorkers to protect themselves For the first time this season, the Health Department has detected West Nile virus in New York City mosquitoes. The infected mosquitoes were collected from four locations in three boroughs: Old Town in Staten Island, East Williamsburg/Bushwick in Brooklyn and the South Jamaica/Rochdale Village and Auburndale/Pomonok neighborhoods in Queens. So far, no human cases have been detected this season. The Health Department has increased mosquito surveillance and mosquito larvae control efforts in the affected areas. Learn more

And then this week, I saw the following at Vos Iz Neias:

The Health Department is reporting unusually high levels of mosquitoes with West Nile Virus in New York City.

Last night, motza'ei Shabbos, there were two mosquitoes in my bedroom, which woke my baby, and bit me and my wife several times. I dispatched them with about twenty minutes of effort, in the middle of the night.

In the past, mosquitoes have caused me and my family quite some trouble. Last night, my arm throbbed from the pain of the mosquito bites. And they often seem to target my wife's joins, such as knuckles, such that it does not merely itch, but hurts. Last year a mosquito bit Junior so many times on the face (it got me as well) that his teachers sent him to the school nurse, who feared he had some sort of chicken pox or the like, and wanted us to take him to the doctor to confirm that he did not have some communicable disease.

May one kill a mosquito on Shabbos, or is it a melacha? My inclination is that it is permitted, and not just because I hate mosquitoes. Rather, I think it is the straightforward reading of the gemara. We pasken like Rabbi Shimon, over Rabbi Yehuda, that for a melacha she'aina tzricha legufa, one is not liable Biblically, but that it is only forbidden Rabbinically. And where an animal is pursuing you and will cause you tzaar, the Sages never instituted a prohibition. And even if they did hold such a prohibition, the existence of West Nile virus, especially where it is prevalent, may make it into safek pikuach nefesh, which according to another reading of the same gemara may make it entirely permitted. (See though my discussion at the end, which could indicate why not.)

The gemara in Shabbos 121 (and here):

ועל עקרב שלא תישך:

א"ר יהושע בן לוי כל המזיקין נהרגין בשבת מתיב רב יוסף חמשה נהרגין בשבת ואלו הן זבוב שבארץ מצרים וצירעה שבנינוה ועקרב שבחדייב ונחש שבא"י וכלב שוטה בכל מקום מני אילימא ר' יהודה הא אמר מלאכה שאינה צריכה לגופה חייב עליה אלא לאו ר"ש והני הוא דשרי אחריני לא אמר ר' ירמיה. ומאן נימא לן דהא מתרצתא היא דילמא משבשתא היא אמר רב יוסף אנא מתנינא לה ואותיבנא לה ואנא מתריצנא לה ברצו אחריו ודברי הכל תני תנא קמיה דרבא בר רב הונא ההורג נחשים ועקרבים בשבת אין רוח חסידים נוחה הימנו א"ל ואותן חסידים אין רוח חכמים נוחה מהם ופליגא דרב הונא דרב הונא חזייה לההוא גברא דקא קטיל זיבורא א"ל שלימתינהו לכולהו

AND {A DISH MAY BE INVERTED} OVER A SCORPION, THAT IT SHOULD NOT BITE. R. Joshua b. Levi said: All [animals, etc.] that cause injury may be killed on the Sabbath. R. Joseph objected: Five may be killed on the Sabbath, and these are they: the Egyptian fly, the hornet of Nineweh, the scorpion of Adiabene, the snake in Palestine, and a mad dog anywhere. Now, who [is the authority?] Shall we say, R. Judah? Surely he maintains, One is guilty on account of a labour not required for itself? Hence it must be R. Simeon, and only these are permitted, but not others? — Said R. Jeremiah, And who tells us that this is correct: perhaps it is corrupt? Said R. Joseph: I recited it and I raised the objection, and I can answer it: This is where they are pursuing him, and is unanimous.

A tanna recited before Rabbah son of R. Huna: If one kills snakes or scorpions on the Sabbath, the spirit of the pious is displeased with him. He retorted, And as to those pious men, the spirit of the Sages is displeased with them. Now, he disagrees with R. Huna, for R. Huna saw a man kill a wasp. Said he to him, 'Have you wiped them all out?'

(See Yerushalmi here, though it does not appear helpful to understanding the Bavli.) There are many ways of understanding this gemara, and specifically the statement by this early Amora, Rabbi Yehoshua ben Levi. It could be that we don't pasken like it, or some of it. But one straightforward reading is as follows.

Rabbi Yehoshua ben Levi speaks of mazikin, which one may kill. This was in accordance with Rabbi Shimon, for we agree with him that מלאכה שאינה צריכה לגופה is patur. That brayta which restricted it to five was originally assumed to be the position of Rabbi Shimon, for Rabbi Yehuda would never permit. Rabbi Yirmeya, an Amora of Eretz Yisrael, tries to defend the position of Rabbi Yehoshua ben Levi, of Eretz Yisrael, by suggesting the brayta is corrupt, and so really Rabbi Shimon would permit in more than these five. Rav Yosef gives another explanation, that the brayta of the five animals which may be killed is discussing where they are actively pursuing him. Therefore it is a case of pikuach nefesh, such that even Rabbi Yehuda would permit in this case. But Rabbi Yehoshua ben Levi is not speaking of a case of pikuach nefesh, and it is going according to Rabbi Shimon, with whom we pasken.

Another way of explaining Rav Yosef's fix it is that Rabbi Yehoshua ben Levi's statement is in accordance with everyone. He is not saying something that is just Rabbi Shimon. Rather, he is dealing with a case in which the animal (other than the five) is pursuing the person, and in such a case, both Rabbi Yehuda and Rabbi Shimon would permit. This does not seem to work well with the give and take of the gemara, which tries to establish the brayta as the position of one Tanna exclusively. But in a pinch, it works, since the initial assumption was that Rabbi Yehoshua ben Levi's statement was within Rabbi Shimon.

However we interpret this, there is also the question of just what sort of mazikin are intended in Rabbi Yehoshua ben Levi's comment, when he says כל המזיקין נהרגין בשבת. The simple reading, to me, is anything which causes any sort of hurt to a person. But given the context of the brayta five animals, and the first interpretation, in which they are pursuing such that it is pikuach nefesh, it is possible that what is meant even in Rabbi Yehoshua ben Levi's statement is that they are dangerous animals that could kill. Given the second interpretation, that Rabbi Yehoshua ben Levi's animals are pursuing him, if Rabbi Yehuda would agree, then perhaps it is because of pikuach nefesh or safek thereof. It all depends on how we cast different statements in the gemara. Even so, I don't think that there is real reason to interpret Rabbi Yehoshua ben Levi's statement as anything other than animals which harm.

See the Rif here:

Five may be killed on the Sabbath, and these are they: the Egyptian fly, the hornet of Nineweh, the scorpion of Adiabene {=a district of Assyria between the rivers Lycus and Caprus}, the snake in the land of Israel, and a mad dog anywhere.
And the remainder of injurious animals, if they are pursuing him it is permitted to kill them, according to everyone {that is, it is not a Tannaitic dispute}.

The Sages learnt {in a brayta}: If one chances upon snakes and scorpions, and he kills them, it is manifest that he had chanced upon them in order to kill them; if he does not kill them, it is manifest that he had chanced upon them that they should kill him, but that a miracle was performed by Heaven on his behalf.
Ulla said: — others state, Rabbah bar bar Channa {our gemara: in the name of Rabbi Yochanan}: That is when they hiss at him.

Rav Yehuda said: One may tread down saliva incidentally {lefi tumo}.
Rav Sheshet said: One may tread down a snake incidentally.
Rav Kattina said: One may tread down a scorpion incidentally.

The Rosh encodes the same. (And Rabbenu Chananel understands similarly.) Thus, the Rif paskens like this gemara, like the brayta, and like Rabbi Yehoshua ben Levi. And he understands "pursuing him" as referring to Rabbi Yehoshua ben Levi's statement. It is unclear why even Rabbi Yehuda would agree. The Ran, like Rashi, says this it speaking about mazikin which kill, such that it is pikuach nefesh. (But maybe we could construct a case similar to tzeda, where the Rambam permits despite holding like Rabbi Yehuda because this is much more than simple מלאכה שאינה צריכה לגופה.)

Rambam writes:

ד  חַיָּה וְרֶמֶשׂ שְׁהֶן נוֹשְׁכִין וּמְמִיתִין וַדַּאי--כְּגוֹן זְבוּב שֶׁבְּמִצְרַיִם, וְצִרְעָה שֶׁבְּנִינְוֵה, וְעַקְרָב שֶׁבְּחִדְיַב, וְנָחָשׁ שֶׁבְּאֶרֶץ יִשְׂרָאֵל, וְכֶלֶב שׁוֹטֶה בְּכָל מָקוֹם--מֻתָּר לְהָרְגָן בַּשַּׁבָּת, כְּשֶׁיֵּרָאוּ.  וּשְׁאָר כָּל הַמַּזִּיקִין--אִם הָיוּ רָצִין אַחֲרָיו, מֻתָּר לְהָרְגָן; וְאִם הָיוּ יוֹשְׁבִין בִּמְקוֹמָן אוֹ בּוֹרְחִין מִלְּפָנָיו, אָסוּר לְהָרְגָן; וְאִם דְּרָסָן לְפִי תֻּמּוֹ בְּשָׁעַת הִלּוּכוֹ וַהֲרָגָן, מֻתָּר.

He considers the five to be certain killers, and one may kill them when he sees them. The others are just termed mazikin, as in Rabbi Yehoshua ben Levi's statement. And it is unclear whether these are potentially fatal or even just hurt him. (If potentially fatal, I would expect explicit mention. But then there is thw word vadai about the five.)  But where they pursue him, one may kill them. A confounding factor is that Rambam is unique in holding like Rabbi Yehuda.

Tosafot has a different explanation of the gemara:

כל המזיקין נהרגין בשבת. אף על גב דתנן במתניתין ועל עקרב שלא ישוך איצטריך לאשמעינן דהריגה נמי שרינן דסלקא דעתך דאסירא משום דמיפרסמא מילתא טפי מבצידה ומהאי טעמא נמי פליג רב הונא בסמוך ואסר הריגה:

ברצין אחריו וד"ה. פי' בקונט' דרבי יהושע בן לוי ברצין אחריו והוי פקוח נפש וברייתא באין רצין וכר"ש וקשה דא"כ תקשה מהך ברייתא לרבא בר רב הונא דשרי להרוג נחשים ועקרבים בשבת דע"כ באין רצין אחריו מיירי דאי ברצין אחריו אמאי אין רוח חסידים נוחה הימנו לפירוש הקונטרס דברצין אחריו איכא פקוח נפש ועוד בניותי שהרג נחש ומיבעיא לן אי שפיר עבד תיפשוט מהכא דלא שרינן לר"ש באין רצין אלא ה' בלבד ועוד קשה לר"י דלישנא דאנא מתרצנא לה משמע לברייתא עצמה ונראה לר"י לפרש דברייתא ברצין אחריו וד"ה פירוש אפילו ר' יהודה מודה דחמשה שרי להרוג ברצין אחריו אבל שאר אסור להורגן אפילו רצין אחריו לר' יהודה ור' יהושע בן לוי כר"ש דשרי שאר אפילו באין רצין אחריו ורב הונא דאסר בסמוך לא סבר כרבי יהושע בן לוי ונראה לר"י וכן פירש ר"ח שאין להקל כרבי יהושע בן לוי אלא כהנהו אמוראי דלקמן דלא שרו אלא בדריסה לפי תומו:

Thus, Rav Huna argues with Rabbi Yehoshua ben Levi. And they understand "pursuing" as modifying the brayta of five animals. The proof is that if otherwise, why in the case of Rava son of Rav Huna did anyone say that the Chasidim weren't pleased? This would, after all, be a case of pikuach nefesh! And so too when they wondered whether one who killed a snake did well. They should have resolved it that according to Rabbi Shimon, only if it were pursuing. Further, the Ri points out that the language of אנא מתרצנא implies the brayta itself. Thus, Ri explains the brayta as even according to Rabbi Yehuda, and it is these five when they pursue. But others (according to Rabbi Yehuda) one may not kill (Josh: but probably just trap, or trod) even when they are pursuing him. And the Ri, and also Rabbenu Chananel (!) hold that one should not be meikil like Rabbi Yehoshua ben Levi, but only like the Amoraim mentioned later, that only when one trods on them lefi tumo, (J:or pretending to be lefi tumo).

I agree with Tosafot about the implications of Rav Yosef's statement. But it seems possible that just as the brayta was speaking about pursuing, so was Rabbi Yehoshua ben Levi, but about non-dangerous animals which just cause pain. After all, both the brayta and Rabbi Yehoshua ben Levi use the term נהרגין בשבת. And Rabbi Yehoshua ben Levi's statement was placed up against the Mishna, where the action was done so as not to be bitten. And then there would be no dispute with Rav Huna. And Rava bar Rav Huna and others were dealing with non-pursuing poisonous snakes and scorpions. And there is no dispute, and Rabbi Yehoshua ben Levi is telling us that according Rabbi Shimon, the Sages never decreed where there was certain or near-certain injury. (It also seems possible that others were unaware of this brayta, or considered it corrupt, like Rabbi Yirmeya, or that they did not agree to their contemporary Rav Yosef's resolution of Rabbi Yehoshua ben Levi's statement. But I am not exploring this here.) Rav Huna is certainly not dealing with a case of pursuing, since he says 'Have you wiped them all out?' implying that what this person has done did not even effectively deal with the situation. And the Amoraim wouldn't (necessarily) be requiring an awkward method of killing which might not succeed in the case where the animal is going to bite and inflict pain. Even if Rabbi Yehoshua ben Levi wasn't saying this explicitly, it is possible that we would still say this ourselves.

The Tur (Orach Chaim 316) brings down the halacha as follows:

He doesn't say that that the five animals kill, but rather they they are מועדין להזיק, though maybe we should not make a diyuk of this. The other damagers, if they pursue, one can kill them actively. And the nachash and akrav (presumably when they are not actively pursuing), one can trod on them. This is in accordance with Rif, Rosh, and Rambam.

Rav Yosef Karo, in Beis Yosef, expands upon this:

" 'Five may be killed in any instance, because they are muad to damage. The fly of Egypt, etc., and the other damagers if they pursue him it is permitted to kill them, etc.' -- this is in the end of perek Kol Kitvei (121b) -- and then he cites the aforementioned gemara; now on to a running explanation-- and Rashi explains "all damagers" {of Rabbi Yehoshua ben Levi} as "who kill", one may kill on Shabbos. And he maintains that even if they are not pursuing him {one may kill these damagers mentioned by Rabbi Yehoshua ben Levi}, because he holds like Rabbi Shimon, that מלאכה שאינה צריכה לגופה is Rabbinic, and here they did not decree.

[Seven lines down]
'{the five animals} May be killed on Shabbos' -- and even where they are not running after him, because he holds that in their stam instance they kill.

'behold he {=Rabbi Yehuda} says one is liable {a sin offering} upon it.' -- and so how could he permit an absolute melacha where they are not running after him, where there is no pikuach nefesh?

'where they are running after him' -- when does Rabbi Yehoshua ben Levi say that all mazikin may be killed? Where they are pursuing him, such that it is an instance of pikuach nefesh.

'And according to everyone' -- even to Rabbi Yehuda. And this {brayta}of five may be killed are where they are not running after him, and it is of Rabbi Shimon.

And Tosafot (there) argue on this explanation and write that it seems to the Ri to explain that the brayta is where they are running after him, and it is according to everyone. That is, even Rabbi Yehuda admits that the five one may kill if they are running after him; but the others, it is forbidden to kill even if they are running after him, according to Rabbi Yehuda. And Rabbi Yehoshua ben Levi is operating according to Rabbi Shimon, that the rest are permitted, even where they are not running after him. And Rav Huna who forbids, in close proximity {in the gemara} does not agree with Rabbi Yehoshua ben Levi. And it appears so the Ri, and so too to R"C (Rabbenu Chananel?} that one should not be lenient like Rabbi Yehoshua ben Levi, but rather one should act like those Amoraim in a bit who only permit when one trods lefi tumo. End quote.

And the explanation of the Rif and the Rosh is like the explanation of Rashi who wrote that five are killed on Shabbos -- and these are they -- the Egyptian fly, etc., and the rest of the mazikin, if they run after him it is permitted to kill them according to everyone. And so it are the words of Rabbenu {=the Tur}, who wrote 'five may be killed in any instance.' And so ruled the Rambam in the 11th perek. And he wrote the same. And this is the position of the Ramban and Rashba in the basics of the law, but wrote that these {five} listed {in the brayta} where they are not pursuing is specifically according to Rabbi Shimon, who exempts by melacha she'eina tzericha legufa, but according to Rabbi Yehuda it is forbidden, because this is not complete pikuach nefesh, since they are not pursuing him. And this is not like the words of Rabbenu {=Rambam}, who ruled like Rabbi Yehuda and ruled here so. And it seems that his position, zal, is that it is pikuach nefesh when one merely sees them. End quote. That is to say, the Rambam explains that when they say 'where they are pursuing him, and according to everyone', that means that where Rabbi Yehoshua ben Levi permits the other mazkin, it is where they are pursuing him and according to everyone, it is permitted. And this that the brayta said that 'five may be killed' is even where they are not pursuing him, and Rabbi Yehuda as well permits it in such a case, because they damage is common, such that it is pikuach nefesh when they merely appear, etc.

And it appears from the words of the Rambam in the 11th perek that these 5 are not exclusive, for such would be the same for any animal or creeping creature which bites and kills for certain. And because of this, when we say 'the other mazikin if they run after him are killed' is where they do not kill for certain, but there is a doubt that they might kill and a doubt that they might not kill. And since there is a doubt that they might kill, where they pursue him it is a safek pikuach nefesh. For if not so, it would not be permitted according to Rabbi Yehuda who declares one liable for מלאכה שאינה צריכה לגופה.

However, according to one who maintains like Rabbi Shimon {that is, other poskim beside the Rambam}, it is possible that even though there is not even any safek pikuach nefesh, it is permitted because of pain. And so writes the Mordechai, and these are his words: 'And the other mazikin' -- that is to say, that do not have any danger -- 'if they are pursuing him, it is permitted to kill them.' And the reason is that he maintains like Rabbi Shimon who said that מלאכה שאינה צריכה לגופה one is exempt for it, and because of the pain, one is exempt for it plus it is permitted even ab initio, just like squeezing the pus from a boil and extinguishing a coal, such that they do not damage him."

This ends Beis Yosef;s commentary in the Tur. It is worthwhile to check out the Mordechai inside. There, it is explicit that this was also the position of Rabbenu Tam, and that it extends to killing even minor mazikin such as the par'osh (louse), though Geonim refrained from killing it in this instance.

In Shulchan Aruch, Rav Yosef Karo writes:

Compare this with Rambam's restatement of the halacha, and notice that it is more or less identical. Thus, he records the version of the Rambam (and like it in reading the gemara, approximately Rosh and Rif), and chooses it over that of Tosafot.

ד  חַיָּה וְרֶמֶשׂ שְׁהֶן נוֹשְׁכִין וּמְמִיתִין וַדַּאי--כְּגוֹן זְבוּב שֶׁבְּמִצְרַיִם, וְצִרְעָה שֶׁבְּנִינְוֵה, וְעַקְרָב שֶׁבְּחִדְיַב, וְנָחָשׁ שֶׁבְּאֶרֶץ יִשְׂרָאֵל, וְכֶלֶב שׁוֹטֶה בְּכָל מָקוֹם--מֻתָּר לְהָרְגָן בַּשַּׁבָּת, כְּשֶׁיֵּרָאוּ.  וּשְׁאָר כָּל הַמַּזִּיקִין--אִם הָיוּ רָצִין אַחֲרָיו, מֻתָּר לְהָרְגָן; וְאִם הָיוּ יוֹשְׁבִין בִּמְקוֹמָן אוֹ בּוֹרְחִין מִלְּפָנָיו, אָסוּר לְהָרְגָן; וְאִם דְּרָסָן לְפִי תֻּמּוֹ בְּשָׁעַת הִלּוּכוֹ וַהֲרָגָן, מֻתָּר

There one striking difference. Where Rambam had it as וּשְׁאָר כָּל הַמַּזִּיקִין, which in Beis Yosef was understood to mean animals which could kill misafek, Rav Yosef Karo changes it to specifically שאינם ממיתין. And he doesn't change it to  שאינם ממיתין ודאי, in contrast to the five in the resha, but rather that they don't kill at all. This makes sense, as a change. After all, as he concluded in his commentary, Beis Yosef, for those who don't hold like Rabbi Yehuda but rather like Rabbi Shimon, not just cases of safek pikuach nefesh, but even where there is no sakana at all. Since we do not pasken like the Rambam in accordance with Rabbi Yehuda, but rather like Rabbi Shimon, this modification is in order. This is readily apparent to anyone who reads the Beis Yosef and then learns the Shulchan Aruch, as one is expected to do. Thus, Rav Yosef Karo paskens like the Rambam but modified to accord with the Mordechai.

The Magen Avraham makes an interesting diyuk in Rav Karo's words in Shulchan Aruch:

In his first comment (22), he is discussing the five animals, and correctly explains that since there is pikuach nefesh, it is permitted even according to Rabbi Yehuda. In his second comment (23), he discusses not the case where mazikin are pursuing him, but the third case where they are not. What one may trod lefi tuma or pretending to be lefi tumo is where they sometimes kill, but not always. He then discusses a שממית called שפי"ן. That is, the spider they call in Yiddish a "shpin". One can just cover the food, and it does not damage humans. Therefore, it is an issur gamur to kill it.

This seems perfectly plausible, given the cases in the gemara of trodding on scorpions and snakes; yet it might well be that things which merely damage but do not kill may also be trodden upon. I am not certain.

Regardless, Magen Avraham does not comment on the middle case, of where they are pursuing him. Thus, he does not ever say that this middle case is not one in which there is not even a safek sakana, but just a case of injury and tzaar. He might hold this, or he might not.

The Chofetz Chaim, in Mishnah Brurah, however, does say what I was saying explicitly. Thus:

Thus, since we pasken like Rabbi Shimon, even if the mazik will only cause pain, the Sages never instituted a Rabbinic prohibition in such a case. This based on Beis Yosef, based in turn on the Mordechai.

With all this in place, let us turn to the case of the mosquito. As I noted, the mosquito causes tzaar. Killing it would be at most a derabbanan, since we hold like Rabbi Shimon. But since it causes tzaar, in some instances at least it one may actively and directly kill it.

This is where it is pursuing you. Does a mosquito pursue? I would claim that it does. True, there are times, between bites, that it sits on the wall, waiting for you to fall asleep of lose attention. But that is laying in wait. And if you try to swat it, it will evade you and move elsewhere. That is not the same as בורחין.

One could read those cases as direct and immediate sakana, such that one has no other choice. But one can also read it as likelihood of damaging, and measuring of the animal's intention. Thus, the five so commonly kill that they need not even pursue. But the others, they would need to pursue. If it was just standing by itself, we don't assume malicious intent, and certainly not if it fled.

But what if a non-rabid wild dog lunged at you, and then stood off at a slight distance, waiting for you to lose focus and lower your guard? Would you call that עומד instead of רץ? Need you wait to kill it until it lunges again? This doesn't seem likely. Similarly, if at the time of your attack the animal retreats, that is not called fleeing, if it will return in a bit if you don't follow through and kill it!

From experience on various Shabbosim, I can tell that moving away a short distance does not help. I have moved to a different room, with the door closed, and the mosquito has followed. And when a mosquito stands on the wall a foot from my head, it is waiting for its next opportunity to bite. I have been bitten often enough to know this.

I would therefore consider this a case of רצין, where there is צער. If so, it should be a straightforward thing to kill it, in according with the Mordechai, Shulchan Aruch and Mishna Brurah.

However, another confounding factor is that it might well be considered like a par'osh, the flea. Though Mordechai notes that even this flea should be included in the permit to kill (or trap -- that is how many understand Rabbenu Tam, though the gemara is talking about killing mazikin), practically, Rabbi Yehuda Gaon reported that the Rabbanei Kashisha didn't even trap it while it was on their skin. (In Shulchan Aruch, previous seif to the one under discussion, we pasken that if the flea is on the skin, one is permitted to trap it though not kill it.)

The question, though, is whether the case of the mosquito is equivalent to the case of the par'osh. I've never had fleas, so I don't know. I was operating on the assumption (quite possibly mistaken -- see here) that the damage done by mosquitoes is sufficiently above that of fleas that we should kill it without a second thought. But I see that flea bites also can be itchy, and produce raised bumps. Though with a flea, it is a simpler matter to pluck it up and remove it, something not as possible (IMHO) with a mosquito which will keep on attacking. (Maybe we could also claim that we are all istenis, and don't tolerate pain as well as they did in the past.) I should really put out a follow-up post, focusing just on the halacha regarding the flea as it developed.

(I would note that I think that the Rishonim were absolutely wrong about the flea, in practice though not in theory. Nowadays, it is indeed harmless. But what the Rishonim did not know is that the Black Death, that is bubonic plague, was quite likely transmitted by the fleas on black rats (though recently this view has been challenged). If this were so, then the flea it is no different from a rabid dog or an Egyptian fly, where there is pikuach nefesh or at least serious safek pikuach nefesh involved. I would say it even likely had the status of the five animals listed in the brayta.)

This is a then simple application of the halacha, since it pursues and causes tzaar. But there is another level to this, which raises the damage even further.

Because many mosquitoes carry the West Nile virus, there is a safek pikuach nefesh involved. And even if not pikuach nefesh, there is an impact of fever, headache, and fatigue. This is certainly mazik. (Indeed, we might even point out that West Nile virus is an Egyptian disease carried by a flying insect, and associate it with the fly from Egypt in the five of the brayta; though West Nile virus is not as worrisome as malaria, for example. If malaria were a concern, then there would be no question that one should kill a mosquito on sight.) If we bring it to the level of pikuach nefesh, then one might kill it on sight. And if we bring it to the level of safek pikuach nefesh, then one might kill it when it pursues even not according to the Mordechai, Shulchan Aruch and Mishna Brurah. And similarly, to trod upon it lefi tumo.

Apparently, killing mosquitoes has been discussed in this context. See this source sheet from a shiur from Rav Pearl. He considers it from the perspective of pikuach nefesh or safek pikuach nefesh.

Thus, (in II - 7) he cites Rav Shlomo Zalman Aurbach that anything the world is worried about because of sakana has the status of sakana. And furthermore, some maintain that safek sakana to the entire community is considered a safek even if it is somewhat sketchy, something not the case for a yachid. If so, even if West Nile virus is not truly so worrisome, since everyone worries about it, and since this is a disease which can impact many, it has the status of safek sakana.

And (in II-8) he cites Rav Elyashiv that one need not fear mosquitoes. However, one who is afraid, it is better to capture it. (Josh: Tzeida is less stringent, in some of the cases cited. In part because this is not only melacha she'aina tzricha legufo, but is actually the very opposite of the usual effect, because one is trying to keep it away from him, rather than getting it.) But that bedieved, it is permitted to kill it. (I am not sure how one would say it is bedieved muttar. Is he saying that if one has no other option, one could do this lechatchila? Maybe that if one killed it, he didn't even violate the deRabbanan. This seems possible.)

I would note that I don't think I am agile enough to catch mosquitoes in this manner. I have enough trouble during the week, during the night, squashing them with books. Usually they evade this fate several times, and in between, hide out of sight or camouflage themselves. If I had to trap each mosquito, I would not get any sleep.

He further writes that he heard that Rav Pinchas Sheinberg ruled similarly to Rav Elyashiv. However, Rav Menashe Klein (of Meshaneh Halachos) wrote that one should not permit in any manner, for the odds are extremely distant, and this is just politics and the media spreading these fears.

I am not convinced about this. It presumably depends upon a person's general health. And so one should not make a blanket statement, including for the elderly (over 50) who are at highest risk for severe cases. But here are the statistics for reported cases of West Nile virus from last year, from the CDC. Recall that this year it is more prevalent in New York. There were 32 fatalities out of a total of 720 reported cases. But there were 373 cases of encephalitis / meningitis, 322 cases of fever, and 25 instances of other symptoms.

Since fever and headache are common and people would not necessarily relate it to a mosquito bite, it is quite possible that it is more common but people did not report. For example, last week Junior had a headache and fever, and was listless all day. I didn't associate it with his mosquito bite, but it is quite possible that it was related.

It seems that all these poskim are treating it from the perspective of pikuach nefesh or safek pikuach nefesh. Perhaps the case put to them was merely seeing a mosquito, or perhaps they analyze the case differently than I do. But I would say that perhaps we don't even need to go so far as to declare it pikuach nefesh or safek thereof.

Rather, analyze it from the perspective of mazik which is pursuing, trying to bite you. It is straightforward in Shulchan Aruch and Mishnah Brurah that one may kill an attacking mazik in such a case. And West Nile, with its various (even non-fatal) symptoms, such as headache, fatigue, and fever, may promote the tzaar from a mosquito to even more to the status of mazik.

Note: As always, this is not intended halacha lemaaseh. Though I am fairly convinced my analysis is correct and straightforward, obviously others disagree with me. Never rely on Internet pesak. Consult your local Orthodox rabbi.