Bacteria may form clumps or colonies. They have no nucleus, and reproduce by dividing. Some are bacteria older than your great-grandfather. Parasite A wide range of pathogens cause such diseases as malaria, schistosomiasis and scabies. These multicellular organisms generally have complex life cycles and must live part of their lives in another organism.
Malaria parasites spend part of their life cycle in certain mosquitoes.
Protein The only infection protein known at this point is the prion. Viruses are between 20 and 250 billionths of a meter in length. For more than you ever wanted to know about these micro-critters. Mucho readings in our bibliography .