Credit: Dr. Libero Ajello/CDC. In addition to viruses and bacteria, there are three other major types of microbes that can cause infectious disease—and one newly discovered type. As with viruses and bacteria, not all of the species in each of these categories are infectious to humans. But many of the world’s most prevalent infectious diseases are caused by microbes that are included in the groupings below. Fungi cause a wide variety of diseases in humans, ranging from Athlete’s foot to ringworm to deadly histoplasmosis.