BUGS

(VIRUSES, BACTERIA AND FUNGI)

Viruses.  These very simple organisms cause most of our day to day infections. They depend utterly on other organisms (humans, animals, plants and even bacteria) to live and reproduce. This is because they have no metabolism (body chemistry) of their own and have to borrow somebody elses. So when you get a cold or flu the virus responsible is using the chemistry of the cells in the lining of your respiratory tract, amongst other places, to build copies of itself. This damages the cell infected which usually dies. With most common viral infections it takes just a few days for immunity to build up before the viruses are all killed. Obviously the virus has got to infect someone else before this happens in order to survive so common viruses are usually very easily passed from one person to another and this is usually by droplet infection (i.e. by coughs and sneezes) or the faecal/oral route. Some viruses have learnt to hide away in cells for many years and either cause recurrent infections(Cold sores and Shingles) or slowly destroy their host cells (HIV/AIDS). Of course the most successful strategy for a virus would be to hide in a host cell and cause no further trouble since making you ill is probably no more in the viruses interest than it is yours except when it causes you to spread the infection to someone else. This almost certainly does happen quite a lot but we are not likely to know much about viruses that cause us no trouble because viruses identify themselves by the illnesses they cause. It is possible that some cancers might be caused by these hidden viruses because we know this happens in animals but so far their isn't much evidence for this happening in humans.

Bacteria. Whilst these are also very simple organisms they are much more complicated than viruses. They have their own chemistry and reproduce independently. Not all bacteria cause infections by any means. Many are totally harmless living independently either in dead material such as the soil or within for example, the bowel, the mouth, the vagina or the surface of the skin. However, some bacteria do always have a tendency to cause infection (e.g. streptococci & staphylococci) whilst others tend to cause infection in debilitated individuals or if they get into the wrong place (e.g. urinary infections are often caused by bowel organisms). Bacteria usually respond to antibiotics but resistance is becoming more frequent these days. Often bacterial infections will be successfully overcome by the body's own immunity.

Fungi. Fungi are much more complex organisms than bacteria and viruses and are on a par with plant and animal cells  in complexity. Most fungi live free in nature and are commonly found in association with dead and rotting material. Not many fungi can invade the healthy body and common fungal infections such as "thrush" and "athlete's foot" and "ringworm" are localised to the body surface. Even "thrush" (also known as candida) requires some weakening of the body's defences such as is caused by antibiotic treatment or by conditions such as diabetes. The "thrush" organism is candida albicans and it normally lives harmlessly in association with us and our "friendly" symbiotic bacteria help to tame it and stop it from causing trouble which is why antibiotic treatment (which kills these bacteria) can trigger an attack of thrush.

Protozoa. You may remember from biology that these are single celled animals. The classic one is the amoeba and indeed there is on species of amoeba which causes a nasty form of dysentery. Malaria is caused by a single celled animal as is the chronic diarrhoeal illness giardiasis. There are a number of illnesses caused by protozoa but they aren't very common in the UK.

Summary. Most common infections are viral. With the exception of the herpes viruses (zoster and simplex) and HIV there are very few for which there is a specific drug. Most of the time this doesn't matter because the illnesses are mild and self-limiting. Bacterial infections are also common but less so than viruses. Some, like meningitis, are very serious and need urgent treatment but the majority of bacterial infections can be coped with by the body's own immunity. Fungal and protozoal infections play a minor part but "thrush", "athlete's foot" and "ringworm" are nuisances whilst malaria is in most cases a debilitating illness but falciparum malaria (found in sub-saharan Africa) is potentially lethal. Amoebic dysentery is also potentially a very severe illness.

Back to: Antibiotics, Sore throat,Infections and Fevers.