Infection is the process of an illness caused by microorganisms such as bacteria, viruses, fungi or parasites. Infections can cause a range of symptoms from mild to severe and sometimes life-threatening. Infection can be spread by direct contact with a tainted person or creature, through the air (droplet transmission), or through touching contaminated surfaces or objects and then touching your eyes, mouth or nose. Some infections, such as the common cold and herpes simplex virus, are caused by viruses.
Bacteria are microscopic organisms that live in the environment and on your skin. There are trillions of bacteria living in your body, and most do not make you sick. However, some can reproduce rapidly and invade healthy cells, causing infection. They can also produce toxins (poisons) that attack and destroy your cells, which can lead to disease.
Viruses are single-celled organisms that can replicate only inside a host cell, and cannot survive outside the body. They can cause a wide range of illnesses from the common cold to influenza and genital herpes. Viruses are responsible for many of the emerging infectious diseases that are a concern today, such as AIDS and SARS.
Fungi are single-celled organisms that can grow in a variety of shapes and colours. They are grouped into categories depending on their appearance under the microscope: spherical bacteria (cocci), rodlike bacteria (bacilli) or curved, spiral-shaped bacteria (vibrio, spirillum or spirochete). Some fungi can be very dangerous and even deadly.