BOOSTING IMMUNITY: A STRATEGY FOR HEALTH-VECTORS

To see how pathogens affect us, let us imagine that you are a perfectly pure person, uncontaminated by any other life-forms. How do you become colonized by hosts of viruses, bacteria, fungi and parasites?

The general answer is that they come into contact with us because we share the same environment; indeed, many of them are an integral part of our environment. Even before we are born, we pick up both pathogens and immunity from our mothers. Once out of the womb, we live together in the same biological envelope of the planet. Pathogens have a variety of ways of getting on and into our bodies. These routes, which may involve a variety of other life-forms, or the contamination of food and drink, are called vectors.

From the amorphous global environment it is sometimes possible to focus right down to a single very specific disease vector. The best-known example is the possibility of worm infestation from undercooked pork; properly cooked meat creates no problem. Our first line of defence against any disease is to disrupt its vectors and prevent the causative organism from getting through to our bodies. To do this we need a broad understanding of the routes used by disease, and the factors which are important in their management.

Pathogens may occupy more than one clear-cut vector, they may spill over from one to others as they become more common, they may adapt and transfer from one to another, as if seeking the best route in. It would be an exaggeration to ascribe such a tactical purpose to them, but in the end this is the picture which develops. Microscopic pathogens are both persistent and prolific; by weight of numbers they are able to fill the opportunities presented to them.

Although some bacteria tend to produce specific patterns of symptoms, this is not always what we experience. It can also happen that one virus attack may be fought off, but the damage caused may allow another condition, previously held in check, to emerge. This is why one disease tends to bring others along in its wake.

There are four fundamental vectors for disease: air, food, drink and other people. These are essential to the continuation of our lives and are constantly open to pathogens. Most infections are caught from the air; we breathe in viruses and bacteria cocooned in microscopic drops of moisture. The usual source of such infections in the air is other people; weather maps show how they may travel vast distances, but their virulence will be affected by the density and distance of the source population. The old health-education poster 'Coughs and Sneezes Spread Diseases' has the virtue of simple truth, but merely sharing the same air is enough to share some pathogens.

Our food can also act as host to infections. When we eat contaminated food we may become infected. The periodic outbreaks of salmonella poisoning bear testimony to the efficiency of this route from the disease's point of view. But it is not a certain means of infection; many other factors are involved apart from ingesting the pathogen.

Similarly, water and other drinks can carry infectious organisms into our bodies. The World Health Organization believes that much of the disease in underdeveloped countries could be eliminated by providing clean water supplies. In countries in decline or suffering population overload, the water supply becomes suspect. This is not always because of biological

contamination; it may also happen because of increasing chemical pollution caused by attempting to 'treat' the water.

Non-biological contamination of these basic vectors is very important because it can have severe effects on our immune systems. Pollution, both overt and covert, in air, food and water, loads the recognition and assessment functions of our immune systems. It is a loading they would do much better without. One sign of the significance of this loading is the growth of allergic responses. These are not infections, although allergic reactions can be precipitated by the effects of pathogens. The vast increase in allergic problems today is one result of contamination by artificial chemical molecules of every part of our global environment. From the environment they end up inside us, causing immune memory overload. The most common response to such overload is an allergic reaction. But even if this does not occur, our immune systems will be compromised in other ways. Pollution of any sort is a fundamental aid to disease.

After air, food and water, the most significant disease vector is other people. Infected people become sources of more infectious organisms, which are returned to the fundamental vectors. Infected people aid the momentum of the spread of the disease. The closer people are to other people the easier it is for airborne infections to spread. Measles, for example, cannot survive in communities where the birth-rate drops below 10,000 per year. If you were given the problem of spreading diseases, modern cities with mass transport, modern offices with air conditioning and large stores would be essential ingredients for success.

Some diseases spread by bodily contact: the close contact diseases. These range from fungus infections spread by skin contact, to sexually transmitted disease, which requires the exchange of contaminated fluids between one body and another.

It is because these routes are so basic to our survival that our protagonists have also found them suitable for their success. Peripheral vectors involve a large element of chance, or bad luck.

Wounds and injuries involve chance. If we are injured, and our skin torn or ruptured, any pathogen in the air has immediate and almost unhindered access to our inner processes. Before our understanding of disease mechanisms, even the most trivial wound could have been fatal. This was true in the fairly recent past, aided by our misunderstanding of the ways our bodies fought infections. During the First World War attempts at antisepsis frequently had more adverse effects on natural immune functions than on infections. The result of excessive sterilization of open wounds was that more of those treated tended to die than those who were not. The best treatment was to change clean dressings frequently; these allowed the immune system to deal with the infection, stopped further invasions, and removed the expelled debris before it became a source of re-infection. As a young doctor, Alexander Fleming, who subsequently discovered penicillin, was involved in the observations which revealed this principle; no doubt its implications deeply impressed him. It was only with the coming of the sulphonamide drugs in the 1930s, and penicillin in the 1940s, that we had effective means of aiding the immune system in its battle against established internal infections.

Infections are also spread by penetrating wounds from animals and plants. Rabies is the

best-known example of disease transmitted via a wound from an animal. The organism passes from the saliva of the infected animal into the blood of the bitten person. Being pricked by a thorn can also transmit disease. Sleeping Beauty and Snow White were probably victims of tetanus bacteria, rather than wicked witches. Such bacteria produce an extremely toxic substance which induces paralysis; this can persist for some time before the body is overwhelmed.

Blood-eating insects, such as mosquitoes and fleas, also spread disease. Before such insects start feeding they usually inject their victim with fluid which will prevent the blood clotting. If the insect has previously dined on an infected victim, it will have picked up some of the infectious organisms, or their eggs, with its meal. Although not all infections drawn into insects are passed on, in some cases they will multiply within the insect, so that the next dinner gets a super dose with the anticoagulant. In other cases, they may be passed on incidentally.

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