Infection occurs when harmful germs (microorganisms) enter the body and multiply. Germs can be bacteria, viruses, fungi or parasites. Some types of infections are very serious and can lead to death. The immune system fights infection with white blood cells and antibodies. Sometimes, this causes symptoms such as fever, chills, sweating and a rash.
Bacteria are single-celled organisms that can survive in many different environments. They can live in extreme heat and cold, radioactive waste and inside the human body. There are trillions of bacteria in the human body, and most do not cause disease. Some “good” bacteria help digest food, fight off disease-causing germs and protect us from infection. Some bacteria, however, can make you very sick, including E coli, Salmonella, Staphylococcus and Clostridium difficile.
Viruses are also single-celled organisms, but they cannot reproduce on their own and must infect other cells to grow. Viruses that can cause illness in humans include influenza, herpes B and herpes C, rotavirus and pneumococcus. Fungi are multicellular and typically do not cause disease in humans, but some can infect skin or soft tissue (for example tinea corporis [ringworm] and tinea pedis [athlete’s foot]). Parasites that can cause disease in humans include protozoa that live in the gut and cause gastroenteritis (for example cryptosporidium and giardia) and helminths, which are single-celled organisms that live on or in other animals and can cause scabies and head lice.
You can get most infectious diseases by directly touching an infected person or animal, inhaling droplets or particles from the person or animal, or consuming or eating contaminated food or water. You can also get infections by getting bitten by an infected insect or by coming into contact with contaminated surfaces.