Infections occur when disease-causing agents not normally present inside the body invade the body and multiply. Infectious (transmissible, contagious, communicable) diseases are illnesses that result from infection. Some well-known infectious diseases are the common cold, influenza, pneumonia, herpes, and HIV. Organisms that cause infectious diseases are called pathogens; they divide into categories that reflect similar evolutionary/phylogenetic characteristics and responsiveness to similar families of medications:

• Bacteria and fungi, both of which are single or multicellular living disease agents that can complete a life span and reproduce independently of a human host.
• Parasites, single or multicellular organisms that depend on a host to survive and replicate viruses, nonliving disease agents that are essentially packets of genetic materials that depend on living host cells to reproduce.
• Prions, which are rare, poorly ...