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Updated on March 23, 2012

The human being can be infected with four totally different types of germs — bacteria, fungi, protozoa, and viruses. Viruses are by far the most common, followed by bacteria and fungi.



Tonsillitis, pneumonia, cystitis, school sores and conjunctivitis all have one thing in common. They may all be caused by bacteria. Bacteria are microscopic single-celled organisms that can penetrate into healthy tissues and start multiplying into vast numbers. When they do this they damage the tissue they infect, causing it to break down into pus. Because of the damage they cause, the involved area becomes red, swollen, hot and painful. The waste products of the damaged tissue and the bacteria spread into the bloodstream, and this stimulates the brain to raise the body temperature in order to fight off the infection. Thus a fever develops.

Millions of bacteria invade the body every day, but few cause problems because the body's defense mechanisms destroy most invading organisms. The white blood cells are the main line of defense against infection. They rapidly recognize unwanted bacteria and large numbers move to the area which has been invaded, to engulf the bacteria and destroy them. It is only when these defenses are overwhelmed that a noticeable infection develops.

Hundreds of bacteria are known to microbiologists (the doctors and scientists who study them), but only a few dozen cause significant infection in the human race. All these bacteria have specific names and can be identified under a microscope by experts who can tell them apart as easily as most of us can identify different breeds of dogs.

When an infection occurs, the victim usually consults a doctor because of the symptoms. If the infection is bacterial, the appropriate antibiotics can be given to destroy the invading bacteria. Because different types of bacteria favor different parts of the body and lead to different symptoms, a doctor can make an educated guess about the antibiotic to use. When there is any doubt, a sample or swab is sent to a laboratory for expert analysis so that the precise organism can be identified, together with the appropriate antibiotic to kill it.

Many bacteria, particularly those in the gut, are beneficial to the normal functioning of the body. They can aid digestion and prevent infection with funguses (such as thrush) and sometimes viruses. Antibiotics can kill these good bacteria too, and common side effects of the use of antibiotics are diarrhoea and fungal infections of the mouth or vagina.

Viruses are a totally different type of organism from bacteria. They are less than one thousandth the size of the average bacterium and antibiotics have no effect on them. Viral diseases, such as colds, flu, measles and hepatitis cannot be treated effectively.

The sooner any infection can be treated, the faster the recovery.

If you think you might have an infection, an early visit to the doctor is advisable.


Mushrooms, the green slime that forms on stagnant pools, and tinea are all related. They are fungi. Fungi are members of the plant kingdom, and are one of the types of microscopic life that can infect human beings in many diverse ways.

The most common site of infection is the skin, where they cause an infection that is commonly known as tinea. The fungus that causes tinea can be found everywhere in the environment in the form of hardy spores. These are microscopic in size and may survive for decades before being picked up and starting an infection. Between the toes the fungus causes a type of tinea commonly known as athlete's foot. This is because athletes sweat and wear close fitting shoes that lead to the ideal warm, damp environment favored by fungi. Similar infections in the groin cause a red, itchy, rapidly spreading rash. In both situations, creams or lotions are used to kill off the fungus before it spreads too widely. The rash is often slow to clear, because the treatments destroy the fungus, and do not necessarily heal the rash. The body heals the rash itself once the infection is control-led.

Unfortunately, fungal skin infections tend to recur because the fungus in its cyst form is resistant to many types of treatment. The active forms of the fungus are killed, but the spores may remain in the skin pores to reactivate once the treatment is ceased. To prevent this condition, keep the affected areas cool by wearing the correct clothing and foot wear, and dry carefully when wet.

Fungi are also responsible for many gut infections, particularly in the mouth and around the anus. It is a rare infant that escapes without an attack of oral thrush. The white plaques that form on the tongue and insides of the cheeks are familiar to most mothers, and this is due to one of a number of fungi. Paints or gels used in the mouth usually bring it rapidly under control.

Around the anus, the fungus can cause an extremely itchy rash, but in women it may spread forward from the anus to the vagina to cause the white discharge and intense itch of vaginal thrush or candidiasis. The movement from the anus to vagina is aided by nylon underwear, tight clothing (particularly jeans), wet bathers and sex.

Fungi live normally in the gut, and are in balance with the bacteria that are meant to be there to help with the digestion of our food. Antibiotics may kill off the good bacteria, allowing the fungal numbers to increase dramatically; or they may migrate to unwanted areas. In these circumstances, they can cause trouble.

The most serious diseases develop when fungal infections occur deep inside the body in organs such as the lungs, brain and sinuses. These diseases are very difficult to treat and it may take many months with potent anti-fungal drugs to bring them under control. Fortunately, this type of condition is relatively rare.

The most obvious connection between the various forms of fungi occurs with the common ringworm. This is not really a worm, but a fungal infection growing outward from a central spore, in exactly the same way that mushroom rings form in the garden in damp weather.


Protozoa are microscopic single-celled organisms like bacteria, but they are significantly larger and closer to what we normally think of as animal-like. Most protozoa are harmless but a few are parasites (i.e. live on a host body) and cause disease, usually of a singularly unpleasant kind. They are found all over the world in the soil and in almost any body of water from moist grass to mud puddles to the sea.

There are various kinds of protozoa, depending on how they travel, either propelling themselves by one means or another, or in the case of the type that causes malaria, having no in-built means of propulsion but relying on a type of mosquito for transport. African sleeping sickness (affecting the nervous system) is caused by a species of protozoa and is transmitted by the tsetse fly, although the organism also has the ability to propel itself with a long whip-like tail.

Other diseases caused by protozoa are various gastrointestinal disorders and infections of the genitals such as vaginitis (inflammation and discharge from the vagina) or urethritis in men. A particularly unpleasant disease is called kalaazar which is transmitted by the bite of a sandfly, and leads to anemia and an enlarged liver and spleen. Another form of the disease attacks the mucous membrane and skin of the nose and spreads to the lips and mouth, causing ulcers, and as it progresses the cartilage of the nose may be destroyed, resulting in severe facial damage.

Toxoplasmosis, which can be transmitted by cats and raw meat, and which can cause fatal or severe damage to an unborn child if a pregnant woman becomes infected, is also caused by a protozoan organism.

Another type of protozoa is the amoeba. This is a sort of irregularly shaped fluid blob enclosed in a membrane. There are several varieties of amoeba, one of which may live in the sockets of the teeth and give rise to gum disease, while others are a cause of brain disease. One variety is the cause of amoebic dysentery, a disorder characterized by severe diarrhea, common in the tropics, and is frequently spread by drinking contaminated water, especially where human excrement is used as fertilizer. It can be an insidious disease in that it sometimes lives harmlessly in the intestines for many years and then, for no apparent reason, invades the intestinal wall and travels to the liver or other organs, where it forms an abscess.


Let your imagination run wild! Imagine an entire world that is only the size of this full stop. A world that has millions of inhabitants, of a thousand or more different species. This is the world of the virus. It can be found anywhere in the environment. It could be in your body, or in a drop of sweat, a particle of dust, or the skin of your family dog.

If the viruses are in your body, they will be under constant attack by the body's defence system. Every minute, millions more viruses enter your body through your mouth or nose. As they enter, the defense system uses its special cells and protein particles (known as antibodies) to repel the attack. Sometimes the defenses are overwhelmed for a short time by the rapidly multiplying viruses. When this happens, you may feel off-color for a day or two. If the virus numbers manage to totally defeat the defenders, you will develop a full-blown viral infection. Viruses can cause diseases as diverse as measles, hepatitis, cold sores, chickenpox, glandular fever and the common cold.

Virus particles are so small that they cannot be seen by even the most powerful light microscope, and special electron microscopes have to be used. They are neither animal nor plant, but particles that are so basic that they are classified into a group of their own. They are not alive in any sense that we understand, but are overgrown molecules that are intent on reproducing themselves at the expense of any host that happens along. Because they are not truly alive, they cannot be killed, and so antibiotics that are effective against the much larger living cells known as bacteria have no effect on viruses. Other than for a limited number of viruses that cause genital herpes, shingles and cold sores, we have no form of cure for virus infections.

The common cold can be caused by any one of several hundred different viruses. They cause the lining of the nose, sinuses and throat to become red, sore and swollen; and phlegm and mucus are produced in great quantities to give you a stuffy head, sore throat and runny nose. The poisons created by the body destroying the viruses circulate around in the blood stream to cause the fever and muscular aches that we also associate with a cold. While you are suffering, the body is busy producing the appropriate antibodies to fight the infection. Once the number of antibodies produced is adequate to destroy most of the viruses, the symptoms of the disease disappear and you recover.

Doctors cannot cure the common cold or other viral infections, but they can help relieve the symptoms with medications to stop the runny nose, clear congestion, and ease the pains. It is also important for doctors to check that the infection has not worsened with a secondary bacterial infection developing in an already weakened body.

Doctors can vaccinate against some viral diseases, such as measles and influenza, to prevent you from catching them; but others such as the common cold cannot be prevented. The much touted vitamin C has not proved itself to be effective when subjected to carefully controlled clinical trials. Viral infections can best be avoided by a good, well-balanced diet, reasonable exercise, avoiding stress, protecting yourself from extremes of temperature, and avoiding those who already have the infection.


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