Many of the micro-organisms that cause illness usually live on us or in us harmoniously causing no harm. We all have millions of different types of bacteria living on our skin, in our throats and in our large intestine. Many are beneficial to us. Some bacteria in the bowel make vitamin K and others help us digest food. They also protect us by keeping out external bacteria that could cause disease.
However even these beneficial bacteria can cause harm if they are where they shouldn't be. Bacteria living harmlessly on the skin for example can cause problems if they enter a wound. Also, some micro-organisms that live harmlessly on some people can bring about disease if they come into contact with others – some meningitis causing bacteria for example.
We have viruses living with us as well. Some reside in our bowel whilst others like Herpes viruses stay dormant in our cells only becoming activated when our immune system is low or the body is stressed in some way. They then appear as shingles, cold sores or genital herpes depending on type.
This symbiotic relationship can break down for a number of reasons resulting in what we recognise as a disease state. The skin is our first defence against infection; when intact it keeps micro-organisms out of the tissues. The skin is also covered with a thin film of oil and this prevents certain types of bacteria infecting the skin.
Mucus, the viscous fluid that coats the inside of the nose, throat, airways and genital tract is another defence against infection.
When micro-organisms break through these barriers to infect our tissues other types of defence come into play. Phagocytosis, part of the inflammatory process, involves white blood cells engulfing the invading bacteria. This process is aided by antibodies. These are proteins which coat and kill micro-organisms.
Our bodies have also developed a specific defence against viruses. When a virus enters the cell it triggers the production of interferon: a substance which destroys many viruses.
Major Micro-organisms Causing Infection
- Viruses: extremely small, they can only be seen with the electron microscope. Basically they are bundles of DNA or RNA wrapped up in a protein coat. They are not killed by antibiotics but may respond to antiviral medication if taken early on in the infection. Viruses are responsible for many illnesses: flu, chicken pox, AIDS, measles and some types of gastroenteritis to name a few.
- Rickettsia and chlamydia: Larger than viruses and can be seen under the light microscope. They generally respond to antibiotics. Examples of illnesses caused by rickettsia are typhus and Q fever. Chlamydia are responsible for a common form of sexually transmitted disease.
- Bacteria: the many types are responsible for a very large number of diseases such as gastroenteritis, sore throat, abscess, tuberculosis and meningitis. They respond to antibiotics, however resistance is a growing problem.
- Protozoa: single celled organisms; an amoeba is an example. They cause diseases such as trichomonas, sleeping sickness and a type of dysentery. They are not killed by antibiotics but respond to other chemical agents.
- Fungi: related to mushrooms and toadstools, candida and athlete's foot are common manifestations. Often arise opportunistically after antibiotics have killed off all the competing bacteria. They are not susceptible to antibiotics but respond to various chemical agents.
This article is for information only. If you have any concerns about your health you should consult your doctor.
Resources
Medicine. Eds. Souhami and Moxham. Pub. Churchill Livingstone 2002.
