when was bacteria as a disease causing agent first discovered
History. Individual bacteria were first seen by humans about 325 years ago when they were magnified by the first microscope. It's only been a little over 100 years since a bacterium was first implicated as a causal agent in a plant disease.Bacteria were shown in 1878 to be associated with fireblight of apples and pears in Illinois and New York, USA (Burrill 1878).(fire blight disease lesson).istory. Individual bacteria were first seen by humans about 325 years ago when they were magnified by the first microscope. It's only been a little over 100 years since a bacterium was first implicated as a causal agent in a plant disease.