Abstract

Infectious diseases can be caused by worms, protozoa, fungi, bacteria, viruses and even proteins (prions). Organisms, viruses and prions can be classified by their infectivity, or their ability to enter, survive and multiply in a host. There are seven classes of viruses, based on their DNA or RNA. By number, 90% of the cells in the human body are bacteria. Even though our lives depend on symbiotic bacteria, it is important that they stay in their proper places in our human bodies, or ecosystems. By the mid-1980s strains of S. aureus emerged which were resistant to common antibiotics. Multicellular parasites include four species of Schistosoma, a flatworm that causes schistosomiasis, which is second in importance only to malaria, with hundreds of millions infected worldwide. In addition to schistosomiasis, helminths can cause ascariasis, dracunculiasis, elephantiasis, hookworm, lymphatic filiaruasis, onchocersiasis, and trichuriasis.