I see where the confusion is coming from. Other hadiths in the same book seem to suggest this is moreso because blood itself is considered unclean. For instance:
> Sahih al-Bukhari 325 (Book 6, Hadith 30) #322
> Narrated 'Aisha: Fatima bint Abi Hubaish asked the Prophet, "I got persistent bleeding (in between the periods) and do not become clean. Shall I give up prayers?" He replied, "No, this is from a blood vessel. Give up the prayers only for the days on which you usually get the menses and then take a bath and offer your prayers".
"Assume good faith" would suggest that at the time the holy texts were written, bleeding was frequently associated with infection and disease. Germ theory came over a millenia later.
> Sahih al-Bukhari 325 (Book 6, Hadith 30) #322
> Narrated 'Aisha: Fatima bint Abi Hubaish asked the Prophet, "I got persistent bleeding (in between the periods) and do not become clean. Shall I give up prayers?" He replied, "No, this is from a blood vessel. Give up the prayers only for the days on which you usually get the menses and then take a bath and offer your prayers".
"Assume good faith" would suggest that at the time the holy texts were written, bleeding was frequently associated with infection and disease. Germ theory came over a millenia later.