The Gemara says that if a woman gets married, then her husband dies; then marries another guy and is widowed from him; then once more; we then assume something about her is causing her husbands to die (three makes a pattern). The woman is considered a קטלנית, katlanit ("killer woman"), and should not marry again.
The Gemara then has two possible explanations to what's going on here:
- Whatever is going on with this woman is metaphysical ("karma", "mazal", whatever), in which case this halacha would apply even if the first husband died of lightning strike, the second of shark attack, and the third of ... well, pick your favorite absurdity.
- There's something medical/biochemical that this woman causes in her husbands. If so, the halacha would only apply if all three died of natural causes.
My understanding is the Gemara is saying "don't try to over-diagnose; even if you don't understand the mechanism, if it's consistently dangerous, stay away!"
Nonetheless, out of curiosity, are there any known medical conditions that could explain the phenomenon?