In the 40 years before AIDS emerged, the world changed more rapidly than at any time since the European conquest of the Americas. Some of the changes were to facilitate the AIDS epidemic, in the same way that social changes in the period following the discovery of the Americas encouraged the spread of syphilis in Europe and numerous infectious diseases in the Americas.

Population movements brought millions to big cities, and economic conditions frequently separated men and women from their partners. A growth in prostitution and polygamy occurred in African cities, and on trade routes between African countries. A dramatic growth in sexually transmitted diseases took place.

Medicine changed dramatically in Africa, with the increased use of injections and transfusions as part of treatment. Owing to lack of resources, needles were not always sterilised or disposed of between patients. Mass immunisation programmes took place without rigorous sterilisation of needles.

International travel made contacts between different countries and continents commonplace. For example, during the late 1970s, an increasing number of European gay men began to visit North America as airline travel became cheaper. Africans visited Europe more frequently, and large numbers of Europeans went to work in Africa. Sexual tourism to destinations such as Haiti and Thailand made a significant contribution to the local economies and attracted visitors from all over the developed world.

Taboos against homosexuality were vigorously challenged, especially in English-speaking countries and Northern Europe. Visible organised gay communities emerged in New York, San Francisco and Los Angeles during the 1970s which attracted gay men from all over the world to visit or settle. It is estimated that 50,000 gay men moved to San Francisco between 1969 and 1982.

Sexual freedom was a primary value and rallying point of these new communities, reflecting a wider sexual revolution in developed countries. Bathhouses and backrooms which allowed for multiple sexual contacts were also a key feature of these communities. Anther new feature of the emerging gay lifestyle was the versatility of sexual roles in an individual's behaviour. Until the 1970s much homosexual activity had been characterised by the maintenance of clear sexual roles of `active' and `passive' in anal intercourse, but as gay men became more open and liberated, this role separation was increasingly questioned and challenged. Fewer and fewer individuals were either exclusively active or passive.

Sexually transmitted infections, antibiotic use and recreational drug use amongst this population were all extremely high, and it was not at all uncommon for gay men in their thirties to have had over one thousand sexual partners in their lifetime. This was a very dramatic change from the limited numbers of sexual partners enjoyed by most gay men before the 1960s (Grmek; Root-Bernstein).

Blood transfusions became increasingly commonplace in medical practice, and an industry grew up dedicated to serving the requirement for both blood and blood products. In the United States paid donors were used - often, injecting drug users. This blood was sent around the world, and US blood donations were sold to Africa during the 1970s.

In the late 1960s hæmophiliacs began to benefit from the discovery that Factor VIII could be synthesised from donated blood. Manufacturers were soon pooling Factor VIII from between 2,000 and 20,000 donors in order to produce Factor VIII concentrate for distribution all over the world. This increased the chance that hæmophiliacs would be exposed to any new infections present in the blood donor population.

Intravenous drug use grew dramatically in the 1970s as heroin became increasingly available in the wake of the Vietnam war. The war introduced thousands of US troops to heroin, and the US government actively aided heroin smugglers in Laos who opposed the Communists, permitting huge quantities of heroin to flood to the West. Wars in Afghanistan and Lebanon also augmented the global supply of heroin, which was processed through Mediterranean ports such as Marseilles and Naples. Large drug dependent communities grew up in Southern European cities which acted as entry points for heroin. Large drug using populations emerged amongst minority populations in US cities, partially as a consequence of the exposure of servicemen to heroin in Vietnam (notably, a disproportionate number of US troops in Vietnam were black). Economic deprivation also led to depression and poverty which found outlets in drug use and drug dealing.

The growth in injecting drug use was also assisted by the arrival of disposable plastic syringes in the early 1970s, and drug-using institutions also developed in some large cities - shooting galleries - where addicts could go to score drugs and rent injecting equipment.

The growth of addict populations also led to an increase in prostitution to support drug habits, amongst both male and female drug users.

All these factors provided pathways for HIV to spread through more and more bodies, generating strains which perhaps became more virulent with each new host encountered.