Infection occurs when microorganisms (usually bacteria, but sometimes viruses or fungus) enter the body and take over the cell or tissue they invade. This can cause illness and may lead to death, depending on the type of pathogen and its effect on host cells. The human immune system is designed to protect against infection, but the microorganisms can overwhelm the immune response and cause disease.
The most common infections are caused by bacteria, viruses and fungus. There are also parasitic infections, which are caused by parasites that live in or on the body of another animal or person and cause disease. Many infectious diseases are spread by direct contact, including the cold sores virus (herpes simplex), influenza and tuberculosis. Some are spread by droplets that contain the germ inside a fluid, such as sneezes, coughs or saliva. AIDS, SARS and tuberculosis are examples of diseases spread by this method. Others are spread by indirect contact with contaminated food, water or objects such as door handles and toys.
Most microorganisms that can cause disease are called pathogens, and they include bacteria, viruses, fungus and parasites. Some of these are ‘good’ and help the body fight infection; others can be harmful and are called ‘bad’ bacteria, viruses or fungus. Generally, the more virulent or dangerous the pathogen, the more severe the infection it causes. Infections can be prevented by washing hands frequently, disinfecting frequently touched surfaces and objects such as phones, counters and doorknobs, practicing safe sex (using a condom during oral, anal and vaginal sex), using bug repellent when outdoors, and covering cuts, burns and wounds with dressings and bandages.