Earlier we stated that most microbes are beneficial; however, some microbes do damage and are potentially harmful and costly. Some microbes cause disease and are called pathogens. Early microbiologists like Louis Pasteur and Robert Koch completed experiments showing that diseases could arise when microbes entered and grew in tissues. Betweend 1875 and 1900 Robert Koch found the causative microbes of at least 20 diseases. This germ theory of disease which identifies the cause of disease-"one microbe, one disease" is still used and important today.

Large populations of microbes may also cause infestations thus reducing the potential productivity of host organisms. Examples of microbes that infest other living organisms include roundworms or nematodes, flukes, tapeworms, and spider mites. These ectoparasites (on surface) and endoparasites (inside host) may infest as many as 50 million people annually in our country. If bacteria increase in high numbers in water, oxygen is depleted and many living organisms like fish may die form lack of oxygen.