Some biologists suggest that greatly increased population densities, unprecedented in human history, may produce the conditions for the spread of both old and new infectious diseases. Many existing human diseases only appeared after the invention of agriculture. Small hunter-gatherer groups did not get many infections.

AIDS is a world problem because it has spread to all countries and affects the whole human community. In modern times epidemics of any kind can spread quickly to all continents because of world transport networks. Cholera is another example as it spread rapidly from Peru in 1991 to the whole of South America. It could be eliminated if money were spent on modern water supplies.

The origin of the HIV (Human Immunodeficiency) virus was unknown for a long time despite intensive search, though it was first identified in the United States from where it seems to have spread to other countries, mainly through infected blood products and sex tourists. Its origin now seems to have been in a virus with the same genetic structure that infects some of the Great Ape species. The method of transmission was almost certainly via butchering the meat of these animals in Central Africa. The Simian Immuno Deficiency Virus does not cause the disease in apes, probably because those animals that had the immunity gene were the ancestors of all those now living. The vulnerable genes have been eliminated, as, eventually, they may be in humans.

One hypothesis was that it may have spread from non-human primates during the development of vaccines against polio during the 1950s. However, no evidence has been found for this theory and it has been discarded.

This article suggests that the HIV virus crossed into human populations in about 1910.

It has long been believed by HIV/AIDS experts that HIV descended from a chimpanzee virus. Although it is believed that the chimpanzee virus was transmitted to people in Africa when the animals were killed for bushmeat, the number of other people who contracted the virus were so few that it did not obtain a durable foothold, according to the AP/Google.com (AP/Google.com, 10/1).

According to the researchers, the spread of HIV increased as a result of urbanization during the colonial era (AFP/Yahoo! News, 10/1). "Cities are kind of ideal for a virus like HIV," Worobey said, adding that urban areas provide the virus the opportunity to spread to other people (AP/Google.com, 10/1). Steven Wolinsky, a study co-author from the Feinberg School of Medicine at Northwestern University, said that increased urbanization during the colonial era meant not only more potential hosts for HIV living closer together but also commercial sex work and other high-risk behaviors (Los Angeles Times, 10/2).

In some countries such as in East and Central Africa the health services are already unable to cope with millions of people now infected.

The concentration of media attention on America and Europe (where it is still a minor though growing problem) has diverted attention from Africa which faces a very serious epidemic - spread almost entirely by heterosexual activity, often along the trucking routes from country to country. Another means of spread is through injection needles in hospitals. Health services can't afford disposable syringes. This was also a major route in Eastern Europe and the Soviet Union which have revealed epidemics of HIV infection. In Romania and the Soviet Union hospital needles have infected many children.

Thailand and Philippines face a similar epidemic. India also faces a major epidemic of African proportions, especially in the urban areas.

The Inuit of Greenland may be facing extinction.

In 1990 WHO estimated 9,000,000 infected people, of which 5,000,000 were in Africa. These numbers could be a considerable underestimate. It is possible that in some areas AIDS could reverse the effects of the population explosion and create the problem of social disruption from the huge numbers of orphans - people lacking the ethical rules derived from the family.

By the end of 1992 the world estimate was 12.9 million, still a probable under-estimate. If there are 110 million (out of 6000 million) HIV cases by 2000 it begins to have an effect on population growth.

By 1994 WHO estimated that in Uganda as many as half those over 15 are infected. An estimate of 5,000,000 orphans in this country alone by the year 2000 was envisaged. The extended family can no longer cope, and even the clan structure may not be able to manage.

This is a situation not met with in Europe since the Black Death (14th century).

Campaigns against HIV

There may be an analogy with a well-studied animal epidemic.

When Myxamatosis was introduced to rabbits in Europe and Australia during the 1950s it spread very rapidly and rabbits were reduced to a few percent of their original numbers. For many years rabbits were rare. But now rabbits are common again. How did this come about?

Myxamatosis kills rabbits rapidly and is spread by a flea. Inside the rabbit burrows the flea could spread easily from rabbit to rabbit.

However, not all rabbits were killed. Those survived which changed their habits to living outside burrows. Presumably some rabbits had always lived out of the burrows but statistically it had not conferred much survival advantage (foxes could catch them more easily).

The present population of rabbits is already close to that before the disease was introduced and is no doubt descended from those which changed the habits. There may have been a change in virulence of the virus too. A virus which killed 100% of its hosts would die out and perhaps the most virulent type is extinct. These are examples of natural selection. The same process applies to humans.

Scientists involved in research into the HIV virus are pessimistic about an early cure or vaccine. There are technical reasons why a vaccine is unlikely without radical new knowledge.

Behavior change
Human history earlier than about 3000 BC is completely unknown. However, it is possible to speculate that the rigid rules of sexual behavior abandoned in the 20th century in western society (on the discovery of penicillin and the contraceptive pill) may have been devised to deal with a similar disease in the past.

It can be imagined that those human societies which adopt similar rigid rules or practice them already will survive, and those which don't will become reduced to a remnant. AIDS does not appear as fast as Myxamatosis and spreads more slowly so the complete cycle would take several human generations.

It seems likely that those societies which practice most promiscuous sexual behavior will experience a serious drop in population. Those communities which practice injection of nonprescription drugs will also tend to eliminate themselves. The population of several African countries seems likely to fall. It seems safe to predict that India will follow. It is already certain that Thailand and the Philippines will suffer a fall in population. Eastern Europe is also at risk from its collapsing health services.

In these extreme conditions, there may be a return to arranged marriages (a custom dormant in the west for several hundred years) with extreme social disapproval of sexual license.

Genetic Immunity
There is some disputed evidence that a few people may be immune (see article on Black Death). If they are, these will be the ancestors of an immune future population, just as the present rabbit population may be partially immune from Myxamatosis.

Treatment
Since this article was first written in the 1990s there have been developments in treatment. Modern drugs allow patients to survive with the virus for much longer with a more or less normal life. In those countries that can afford treatment, being infected is not an immediate death sentence - though it is too soon to know how long the current treatments will remain effective, as the Virus has the ability to change its structure to combat some of the drugs. Countries such as South Africa where the government (President Thabo Mbeki) refused to use the drugs may have suffered a loss of life that could have been avoided. In many other countries in Africa the cost of the drugs is still too high for most people to use them.

In many countries education about the virus has been ineffective and many people apparently still believe the disease to have other causes, such as "evil spirits".