What did these doctors conclude about the main cause of the outbreak of the Black Death?

Bacteria

Explanation:

Not sure which time periods doctors you are referring to, but modern day scientists have concluded that the bubonic plague, or black death was caused by a bacteria called  Yersinia pestis. This bacteria originated in asia and was spread to europe via ship and trade routes. The bacteria was carried by the lice and fleas on rats, which carried the plague onto ships and around the world.

answer: born in saint-dié, in the vosges department, france, he studied law, and was called to the bar at paris in 1854,[2] but soon went into politics, contributing to various newspapers, particularly to le temps. he attacked the second french empire with great violence, directing his opposition especially against baron haussmann, prefect of the seine department. a series of his articles in le temps was later republished as the fantastic tales of haussmannlaxupa (1868).[2]

explanation:

general allotment act (dawes act) february 8, 1887

amendment to the dawes act, february 28, 1891

commission to the five civilized tribes (dawes commission), march 3, 1893