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Over the centuries epidemics of various kinds have decimated populations. A particular epidemic of which there is substantial historical record was the 1918, 1919 influenza-pneumonia pandemic that affected North America. It has been estimated that 20 million people suffered from the illness in the United States and 850,000 died. On a world scale it has been estimated that during that epidemic 20 million people died. Prisoners and military personnel were used as guinea pigs when attempts were made to deliberately infect the subjects to produce the disease. In vain. Only very few developed influenza, no deaths!

What do we mean by infection?

Dorland’s Medical Dictionary defines infection as:
1. Invasion of the body by pathogenic microorganisms and the reaction of the tissues to their presence and to the toxins generated by them; often applied to the presence of microorganisms within the tissues whether or not this results in detectible pathological effects.

2. A general term applied to the invasion of the body by bacteria, viruses, protozoa, metazoan, helminthes etc.

So called infectious agents, that is the life forms that may successfully invade the tissues and gain entrance to the body, range enormously in size from the smallest virus which may be no larger than 20 nanometres (.0000008 inches in diameter) to the parasitic tapeworm that may be as long as 10 metres (33 feet). These various agents are categorized as viruses, rickettsiae bacteria, fungi and parasites. Many of these agents, in fact most of them, do not require a human host for their survival and many will live in soil, water, other animal species are harmless to humans.

What are the similarities between infection and disease?

There are other living organisms, which require warm-blooded animals and flourish on the skin or in the mucous secretions of mouth and/or intestinal tract and under average conditions do not invade the tissues. So it is important that we understand the distinction we are making between infection and disease. All animals are infected to some extent with biotic agents and this does not overstate the facts. Many biotic agents are not involved in the process of disease and are termed non-pathogenic or commensal. Those that are related to disease and believed to be a cause with a primary or secondary role are termed pathogenic. If we were to take a swab from the throats of a random group of people we would probably find that more than 90% were hosts to streptococcus viridians. In the throats of healthy people they are not “pathogenic”. However under very different circumstances, if the same organism is cultured from the blood, that is a very different matter. It is considered to be highly pathogenic and commonly indicates the presence of acute bacterial endocarditis where the valves of the heart are providing a medium for the bacterial growth and proliferation.

Does infection always lead to disease?

Now obviously for such non-pathogenic organisms to become major pathogens they require an ally. They must overcome the defence mechanisms of the host. There must be a portal of entry either through the intact skin or mucous membrane. They must also be a sufficient number to survive the body’s phagocytes (a special kind of white blood cell), which would certainly be mobilized to destroy them. Furthermore they must survive the vigorous specific immune response of the host organism. So we can readily see that if the biotic agent is capable of invading the tissues and proliferating the conditions of the host is of great concern. Under normal circumstances this would not happen and it is easy to understand why when one studies the course of events that occurs from infection to disease why it is so easy to neglect the contribution of the host and emphasize the so-called virulence of the microbe. In any interaction between host and parasite the result is not invariably predictable. Individual variation determines the results. Infection occurs but it is not foregone conclusion that disease must arise.

We are all carriers!

In each individual case the interaction between host and parasite is variable. Some hosts display typical symptoms of disease and infection is easily recognized. Some show no symptoms of disease whatsoever. There is in fact apparent and inapparent infection. These inapparent infections are often referred to as “carrier states”. It may not be very comfortable for people to acknowledge that we are all infected one way or another but this is indeed a fact.

Microorganisms are important …

It is also a fact that such infections are necessary to our normal survival. This is probably more difficult for people to appreciate. Many years ago it became possible to produce animals in a completely germ free environment and have them reproduce under those conditions. This certainly demonstrated that life was not absolutely dependent on an association with microbes. However, it was revealed that animals that have grown and developed under germ free conditions are abnormal in many respects and these abnormalities are sufficient to demonstrate that they could not survive in an ordinary environment and certainly could not compete with animals raised under natural conditions. Their nutritional requirements are much more demanding. Their susceptibility, so-called, to infection much greater; they are abnormally affected by even mild stress. So if you look at the total picture, you should be glad you are infected.

… but can turn nasty at times

So it is not difficult to demonstrate that microorganisms are important to our growth, development and health and only when we undermine, significantly, the integrity of the host does infection really become a problem. However, this does not mean that we should become blasé and think, as some do, that the biotic agents, viruses, bacteria and the like, function as nothing but scavengers for the host removing unwanted material, breaking down cellular debris and the like. No, the microbes are complicating factors. Even though we question their role as the cause, we must conclude that they are part of the cause. The microbes may fulfil certain useful purposes but they also contribute significantly to the pathology.

… like cholera …

Let’s look at an example. In cholera, death is commonly due to massive depletion of electrolytes in water. A toxin produced by vibrio cholerae attaches to the cells of the intestinal wall called enterocytes and stimulates the production of an enzyme, adenylate cyclase. This seriously interferes with an important intra cellular enzyme, cyclic AMP (3-5-cyclic adenosine monophosphate synthetase) consequently disrupting the sodium pump mechanism allowing potassium and bicarbonate to bleed from the cell, a very serious consequence. Understanding this mechanism, oral and intravenous administration of potassium rich electrolyte solutions has become common in the treatment of cholera in recent years the death rate has markedly reduced.

… or the 1919 influenza pandemic.
Let us revert to post-World War I epidemic of influenza pneumonia:

Of course in hindsight we are aware that the high mortality rate was related to widespread malnutrition; but a great deal has been made of the fact that the primary infection was viral but was complicated by secondary bacterial infection. Which leads us to the single most important factor determining if an infection leads to disease: Our state of health.

A question of health!

The body’s response to infection is generally defensive and how effective it is will depend upon many factors, which are related to the health and integrity of the host. Our focus should primarily be to elevate the health of the individual, by genuine hygienic means, so that should infections occur that could lead to something serious the body’s defence will be prompt, vigorous and effective.

To be continued

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