A: Bacteria: Bacteria exist everywhere, inside and on our bodies. Most of them are completely harmless and some of them are very useful. But some bacteria can cause diseases, either because they end up in the wrong place in the body or simply because they are 'designed' to invade us. Bacteria can infect humans, plants or animals.
B: Viruses: A virus is a small infectious agent, too small to be seen by the naked eye. They can't multiply on their own, only inside the cells of other organisms. Viruses consist of genetic material ( DNA or RNA) surrounded by a protective coat of protein. They are capable of latching onto cells and getting inside them. The cells of the mucous membranes, such as those lining the respiratory passages that we breathe through, are particularly open to virus attacks because they are not covered by protective skin.
C: Fungi: Any of a group of unicellular, multicellular, or syncytial spore-producing organisms feeding on organic matter, including moulds, yeast, mushrooms, and toadstools can be either moulds or yeast. A mould which causes infections in humans is Trichophtyion rubrum which is one cause of ringworm and it may also infect nails. A common yeast infection is thrush caused by Candida albicans.
D: Parasites: Are organisms such as fungi or worms that live in or on a host to obtain nourishment without providing any benefit to the host. Worms are not always microscopic in size but pathogenic worms do cause infection and some can spread from person to person.
Common illnesses and infection caused by bacteria, viruses, fungi and parasites are:
Influenza ( Flu )
Measles or Rubella
Colonisation is the presence of bacteria on a body surface ( like on the skin, mouth, intestines or airway) without causing disease in the person. The infected person may