Which scientist created tests that confirm that bacteria and other microorganisms cause a variety of diseases? robert koch

Yes, you are correct.
It was Robert Koch who created tests that helped confirm that bacteria and other microorganisms cause a variety of diseases. Before that, the scientists were not as sure as after they have solid proof of what causes these kinds of diseases. 

answer: - b cells

explanation: -

there are many types of white blood cells present in our body. one such class of white blood cells are the lymphocytes. the b cells belong to the lymphocyte class of wbcs. the b cells perform their action against the pathogen by producing specialised proteins called as antibodies. these antibodies are produced as and when the b cells encounter pathogens in response to the antigens present on the surface of the pathogen. these antibodies bind to the antigens present on the pathogen and then, target them for destruction by other cells of the immune system.

so, the b cells mediate their action by identifying the antigens present on the surface of the pathogens and then producing antibodies in return to mediate its effect.