What type of plague will cause community leaders to declare a public fast?
The Gemara relates that Rav Yehuda was informed that a plague had broken out among the pigs in the community. He responded to the report by calling for a public fast. The Gemara rejects the suggestion that Rav Yehuda believed that a plague among one type of animals could transfer to others and thus posed a danger to humans, arguing that he saw the case of pigs to be unique, since the intestines of pigs are similar to human intestines.
There is no doubt that some types of diseases can be transferred from animals to people. Trichinosis, for example, is a disease carried by pigs that can be transferred to humans, although that ordinarily takes place only if the flesh of the infected animal is eaten, which was not a concern in our case. Nevertheless, there are similarities between the internal anatomy of pigs and humans that are known to scientists today. These similarities lead to use of the intestines of pigs in human transplants, due to a relatively small incidence of rejection of such tissue. Rav Yehuda’s concern was that in this specific case, these similarities may lead to the transfer of the swine plague to people.
Based on this discussion, Tosafot take for granted that if a plague breaks out among non-Jews, the Jewish community would declare a fast, since the possibility of the plague spreading from non-Jews to the Jewish community seems to be obvious. Surprisingly, the Ritva disagrees with Tosafot. The Meiri argues that our Gemara really means to teach that a plague among idol worshippers should be seen as potentially dangerous to the Jewish community, and the Gemara discusses pigs as a code word for idol worshipers, based on the passage in Tehillim 80:14.