Infectious diseases are any of those pathological conditions spread among all biological species. Infectious diseases, although varied in their effects, are always associated with viruses, bacteria, fungi, protozoa, multicellular parasites and aberrant proteins known as prions. A complex series of steps, mediated by factors contributed by both the infectious agent and the host, is required for microorganisms or prions to establish an infection or disease. Worldwide, infectious diseases are the third leading cause of human death.