Question

finally, it was determined that fleas on rats caused the black death, but that wasnt until the 1850s 1890s 1950s 1990s

Explanation:

The Black Death was a devastating pandemic. Historical research shows that it was in the 1890s when it was determined that fleas on rats were a key factor in spreading the disease. This was based on scientific investigations and understanding of disease vectors during that period.

Answer:

1890s