Infection happens when microorganisms (like bacteria, viruses, fungi and parasites) get into the body and cause illness. Many of these organisms are common and normally harmless or helpful, but under certain conditions they can become disease-causing. Usually, these disease-causing organisms, also known as pathogens, get into the body through the mouth, eyes or nose or from wounds or bites that break the skin. These organisms can also enter the body through contaminated food or water or from infected animals or insects (called vectors) like fleas, ticks and mosquitoes that transmit diseases such as malaria, yellow fever and encephalitis.

Some diseases spread through direct contact, such as touching infected skin or mucous membranes. Others spread through indirect contact with contaminated objects or surfaces, such as doorknobs, countertops, and faucet handles. Some infectious diseases, such as herpes viruses and sexually transmitted infections (STIs) such as gonorrhea and chlamydia, are spread through airborne transmission from droplets that contain the organisms emitted when an infected person coughs or sneezes.

Bacteria are the most common cause of infection. They have evolved to adhere to cells, make paralyzing poisons and other toxins, evade or suppress the immune system’s antibodies, and resist drugs. Viruses are also common causes of infection, but they are quite different from bacteria. They can spread rapidly, causing pandemics such as influenza and the bubonic plague (also known as Black Death, caused by Yersinia pestis bacteria) and smallpox, which was caused by the variola virus. Fungal infections are less common but can be serious, such as ringworm and candida.