Infection is the presence of harmful germs (also called pathogens) in an ordinarily sterile part of the body or on a normally clean surface. Infection can cause disease, and symptoms depend on the type of germ involved and the part of the body affected. Symptoms also vary depending on the immune system response.

Bacteria are single-celled organisms that can reproduce on their own and survive in a wide range of environments. They’ve been around for millions of years, evidenced by fossils. They’re found in soil, water and in the human body. Most bacteria aren’t harmful, but some can be. They can cling to skin, enter the bloodstream and invade other parts of the body where they don’t belong, such as the lungs, heart, brain or intestines, causing infections like impetigo or boils. Harmful bacteria can also spread through contaminated droplets in the air (like those produced by sneezes, coughs or breathing) and from animals that carry them on their fur, scales or feathers, or in faeces.

Viruses are even smaller than bacteria, with just a protein coat and a core of genetic material, either DNA or RNA. They can’t survive on their own, so they need living cells to grow and multiply. Viruses can cause serious and sometimes life-threatening diseases, such as the bubonic plague (Black Death) and smallpox. Like bacteria, viruses can also infect other living things, including humans, causing illnesses such as colds and flu. Other infectious organisms include fungi (which can cause infections such as yeast infection and ringworm) and protozoa (which can cause infections such as amebic dysentery). These organisms can only be transmitted through direct contact with an infected person or animal, or through a vector, such as a tick or mosquito.