Finally, it was determined that fleas on rats caused the Black Death, but that wasn’t until the

1850s.
1890s.
1950s.
1990s.

1890s.

Explanation:

Two bacteriologists, the Japanese Shibasabura Kitasato and the Swiss Alexandre Yersin, were able to isolate the plague bacillus (Yersinia pestis or Pasteurella pestis) in 1894.The infection is transmitted from wild rodent fleas (Xenopsylla cheopsis), and human flea (Pulex irritans) almost never becomes a vector. The incubation lasts from two to fifteen days, although usually four to five days, depending on the form of the disease.