Dear Secretagent,
The explanation of the tosfos and the black plague is thus:
The black plague, in addition to killing many millions across Asia and Europe, was especially devastating to pig livestock,what chachamim and others called “dever”, pestilence among the livestock .To a large degree, the pig population infected the human population. Needless to say, this affected goyim on a large scale, but relatively few Jews.
The halachic question then arose, was there enough danger to Jews to warrant a ta’anis? On the one hand, Jews were not really being affected much. On the other hand, since according to the gemara, pig intestine is similar to human intestine, then,certainly a non-Jewish human intestine would be similar enough to a Jewish intestine to justify alarm, and hence, a ta’anis.
Note that tosfos uses the phrase, “Umikan nireh”, a phrase used when tosfos is extrapolating from the gemara to a current halachic decision. So it appears that tosfos is dealing with an actual case where the ta’anis decree was being considered due to dever- the black plague raging at that time, 1348-1353 or so. Since the circumstances of the black plague fit this scenario, this tosfos is considered to have been written around that time, the mid 1300s.
I’ll just add that the actual microbiological cause of the black plague is being debated even today, so the above conclusion is not absolute proof. But it fits.