What are infectious agents in the chain of infection?

For a health care-associated infection to be transmitted there first has to be an infectious agent. The infectious agent, or pathogen, is the organism that causes disease. Types of pathogens include the following:

• Viruses: Viruses are pathogens that grow and reproduce by attaching themselves to a host cell. Viruses can reproduce exactly as they were or can mutate into slightly different versions in each person they infect. Viruses include the common cold, the flu, hepatitis, and human immunodeficiency virus (HIV) and cannot be treated using antibiotics.

• Bacteria: Bacteria are single-cell microorganisms, most of which are not harmful and are often beneficial to humans. Other bacteria like those causing strep throat, Lyme disease, tetanus, and tuberculosis are harmful and can grow, reproduce, and spread throughout the body. Most bacteria can be treated using antibiotics, but some strains, including strains in health care settings, are able to resist antibiotics, making them very difficult to fight.

• Fungi: Fungi like mildews, molds, yeast, and mushrooms are organisms that feed on organic matter. Like bacteria, fungi are mostly harmless, but some can be harmful and cause infections of the skin, nails, lungs, internal organs, and, in some cases, the entire body, including diseases such as athlete’s foot, thrush, and ringworm.

• Parasites: Parasites such as roundworms, protozoa, tapeworms, and flukes are organisms that live in or on another organism. They take nourishment from their host and provide nothing of benefit in return. Parasites commonly enter the body through contaminated food or water, insect bites, and sexual contact and can cause diseases such as scabies, malaria, giardia, and toxoplasmosis.

• Prions: Prions are the smallest of the infectious pathogens and have no genetic materials of their own. Prions are made up entirely of protein material that can fold itself into abnormal structures and influence proteins in a host usually proteins found in the host’s brain to fold up into equally abnormal structures. Prions are rare but can progress rapidly and are always fatal. Prions can cause diseases such as Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy, otherwise known as mad cow disease, in animals.