Was Rav Yehuda concerned about swine flu, or about hog cholera?

Life in Israel has a post about two interesting statements from the chief rabbinate of Israel. The first concerns bonfires on lag baomer, and to be careful, and also:

all should remember that wood should not be taken from building sites or private property, but only from hefker.

In terms of this particular aspect, I am reminded of the custom of bonfires after Succot, and how some kids took the belongings of others to make the bonfire, and steps taken against that.

In a second statement, he reports that:

Chief Rabbi Shlomo Amar has issued a statement in response to the Swine Flu (or Mexican Flu if you prefer). According to Ladaat.net, Rabbi Amar has called upon everyone to accept this Thursday as a day of fasting and prayer. He quotes the gemara that says when there is illness by the pigs, they declared a fast day - because the digestive tracts of the swine are similar to that of humans. As well, when there is a plague in one place, they should fast in other places as well, as the plague can spread from one place to the other easily.

Both of these are based on the gemara in Taanit 21b. To quote Ladaat's summary:

שאמרו לו לרב יהודה, שיש מגיפה בחזירים, ומיד עמד וגזר תענית על הצבור, משום שבני מעיים ומערכת העיכול של החזירים דומה למערכת העיכול של האדם. עוד אמרו שם שאמרו לשמואל שיש מגיפה במקום רחוק, וגזר תענית, משום שזה עובר ממקום למקום.

Earlier, we saw this gemara as applying to swine flu at matzav.

However, I believe that this is a misguided derivation from the gemara. If we examine the gemara closely, we see from the give and take that these were medical concerns. The gemara wonders whether Rav Yehuda is of the position that if a disease is cast on one species, it is cast on all species. And the answer is that it is not so, but just because both pigs and humans have similar digestive tracts (Rashi: that they lack a rumen).

If so, it is medical concerns. And do we really hold like the medicine of the gemara?! Surely not! See Rambam. Rather, we determine the medical reality, and then apply whatever halacha is to apply.

If swine flu is indeed dangerous and a fatal pandemic, then whether or not there is an explicit gemara, it is appropriate to declare a fast day. And if it is not, then the gemara is frankly irrelevant. The same for Shmuel and a disease far away. What do medical professionals tell us about the possibility of this spreading in virulent form. Because in mild form, it is no different than human influenza in its effects.

Declaring a fast day, while having the positive effect of getting people to beseech Hashem for mercy, has the possible negative effect of sparking hysteria in an already some-times ignorant public (especially if that public eschews secular knowledge, as found in e.g. newspapers). On the other hand, it does show that Jews are concerned for the world at large.

So what of the gemara? Is it discussing a pig disease that may spread to humans?

אמרו ליה לרב יהודה איכא מותנא בחזירי
גזר תעניתא
נימא קסבר רב יהודה מכה משולחת ממין אחד משולחת מכל המינין
לא שאני חזירי דדמיין מעייהו לבני אינשי
אמרו ליה לשמואל איכא מותנא בי חוזאי
גזר תעניתא
א"ל והא מרחק
אמר ליכא מעברא הכא דפסיק ליה

What is the disease which is fatal for pigs, which is מותנא בחזירי? There are two candidates, swine flu and hog cholera. Swine flu is not really a pestilence for pigs. Rather,

In pigs influenza infection produces fever, lethargy, sneezing, coughing, difficulty breathing and decreased appetite.^[14] In some cases the infection can cause abortion. Although mortality is usually low (around 1-4%),^[1] the virus can produce weight loss and poor growth, causing economic loss to farmers.

I would not expect this to be described in the gemara as מותנא בחזירי. Furthermore, why talk about the digestive tract being similar to humans? I suppose decreased appetite can fit.

But here is the other candidate, a completely different disease: acute hog cholera. Hog cholera is indeed often fatal for pigs. To cite:

Hog cholera (HC) is a highly contagious viral disease of swine that occurs in an acute, a subacute, a chronic, or a persistent form. In the acute form, the disease is characterized by high fever, severe depression, multiple superficial and internal hemorrhages, and high morbidity and mortality.

also,

Pigs of any age may be affected. There are typically a high fever, loss of appetite, and dullness. Other symptoms include blotchy discoloration of the skin (particularly the extremities), incoordination and weakness of the hindquarters, constipation followed by diarrhea, gummed-up eyes, and coughing. Death occurs within 4–7 days, and the mortality is usually high.

This is something easily described as מותנא בחזירי.

What about the similarity of digestive tracts? Well, constipation and diarrhea are symptoms of hog cholera, and

Hemorrhages are also found on the surface of the small and large intestine, the larynx, the heart, the epiglottis, and the fascia lata of the back muscles.

One could easily worry that it would manifest in humans, as cholera, which also affects the digestive tract, as its "action on the mucosal epithelium lining of the small intestine is responsible for the disease's infamous characteristic, exhaustive diarrhea."

Was Rav Yehuda correct to be concerned? Well, if swine flu, it can exist in mild or extreme forms, and it is zootonic, which means it can cross species and impact humans. However, as I will discuss soon, the most virulent forms cannot be widespread to humans, unless they are really packed together, for reasons having to do with evolution. (To be discussed a bit later.) Besides, it is likely not the swine flu, because that is not so drastically fatal to pigs, as discussed above.

What if hog cholera? In that case, it is fatal to pigs, in high numbers. And it is also zootonic. However, while humans can carry hog cholera, and thus help infect other pigs, it is not harmful to humans.

I would note that while the gemara notes that Rav Yehuda was concerned and thus declared a fast, we do not have evidence that it was in fact harmful to humans. Rav Yehuda was in all likelihood working with the best medical knowledge of his time.

On to the concerns of the virulent forms of the disease spreading from elsewhere, as per Shmuel's concern. I read an interesting article in the New York Post, which for some reason I cannot find online. Note that the fatal cases are all centered in Mexico, at the point of origin, and the one case in the US was an infant from Mexico. Other cases are milder.

The author of that article pointed out a reason for it. For the virus to spread, people have to be manifesting the effects of it and moving about. The most virulent strains cause people to be bedridden and then die in pretty short order. This means that they are not going about in public, spreading it. Meanwhile, the milder strains, which have basically the same effects as human influenza which is going around anyway, allow people to go out in public and spread it. So yes, it might spread a lot, but there are natural limits, imposed by the evolutionary mechanisms, which seem to be ensuring that this virus stays mild as it spreads.

Update: See also the discussion of the Meiri and whether this was metaphorical as noted by anon1, but also at length over at DovBear.