Daf Yomi Yevamot 64b: Chazaka, Epilepsy, and Leprosy

In about a week, we will encounter the following gemara in Daf Yomi (translation from a Rif Yomi post in preparation):

{Yevamot 64b}

אמר רבא לא ישא אדם אשה לא ממשפחת נכפין ולא ממשפחת מצורעים והוא דאיתחזק בתלתא זימני אבל בתרי זימני אקראי בעלמא הוא:

Rava said: A man should not marry a woman from a family of epileptics or from a family of lepers. And this is where it has been established from three times. But from two times, it is mere happenstance.

The idea is perhaps that we establish all things, including likelihood of physical malady, with chazaka, and this is no exception. Of course, if so, why give this justification that "it is mere happenstance." Of course! Why give this justification.

I would also point out that there is a possible distinction between the first and second half of Rava's statement. The first part is ambiguously Hebrew. It might pass for Aramaic, though we might anticipate inish and iteta rather than adam and isha. But, the second half of the statement, which clarifies with the word והוא, is unambiguously Aramaic. Thus the de prefix and the Aramaic vocabulary. So Rava might not have explicitly stated this, and this is the clarification of the setama digmara.

Is there reason to worry about a woman being from a family of epileptics or of lepers? That is, it is genetic? And if it is genetic, then shouldn't one instance be sufficient. (Absent genetic testing. And we assume here that these translations are accurate, which admittedly may not be so.)

It turns out that both epilepsy and leprosy are both genetic and developmental. For epilepsy (Wikipedia source):

Epidemiology

Epilepsy is one of the most common serious neurological disorders.^[9] Genetic, congenital, and developmental conditions are mostly associated with it among younger patients; tumors are more likely over age 40; head trauma and central nervous system infections may occur at any age.

and for leprosy:

Leprosy or Hansen's disease is a chronic infectious disease caused by the bacterium Mycobacterium leprae.^[1] Leprosy is primarily a granulomatous disease of the peripheral nerves, mucosa of the upper respiratory tract, and skin lesions.^[2]

...

Pathophysiology

The exact mechanism of transmission of leprosy is not known. The only other animals besides humans to contract leprosy are the armadillo, chimpanzees, sooty mangabeys, and cynomolgous macaques.^{[citation needed]} The bacterium can also be grown in the laboratory by injection into the footpads of mice.^{[citation needed]} There is evidence that not all people who are infected with M. leprae develop leprosy, and genetic factors have long been thought to play a role, due to the observation of clustering of leprosy around certain families, and the failure to understand why certain individuals develop lepromatous leprosy while others develop other types of leprosy.^{[citation needed]} However, the role of genetic factors is not clear in determining this clinical expression. In addition, malnutrition and possible prior exposure to other environmental mycobacteria may play a role in development of the overt disease.

Thus, epilepsy may be genetic, but they may also be the result of other factors. And leprosy might be the result of a certain bacterium, but genetic factors might make one more susceptible.
Thus, one occurrence is not really enough to establish a genetic trait in the family. Perhaps it was from some other cause. Two as well, especially if we are considering a somewhat extended family. With three in the same family, it stands to reason that there is some genetic trait being manifest, and there is what to worry about.