Negative roles
Some prokaryotes are human pathogens i.e they cause about half of all human diseases. For example, Lyme disease is caused by a bacterium and carried by ticks. Pathogenic bacteria pathogenic prokaryotes typically cause disease by releasing exotoxins or endotoxins. Exotoxins are secreted and disease even if the prokaryotes that produce them are not present. Endotoxins are released only when bacteria die and their cell walls break down.
Positive roles
Prokaryotes are the principle agents in bioremediation, the use of organisms to remove pollutants from the environment. Bacteria can be engineered to produce vitamins, antibiotics, and hormones. Still bacteria are also being engineered to produce ethanol from waste biomass