Research published today in Science shows how there has been an almost 50 percent decline in HIV prevalence in some groups, which the researchers attribute to people delaying when they first have sex and having fewer casual partners.
They found HIV prevalence fell most steeply at young ages, with a drop in prevalence of 49 percent for women aged between 15 and 24, and a 23 percent drop in men aged 17 to 29.
In 2003 Zimbabwe was estimated to have 1.8 million people infected with HIV/AIDS out of a population of 12 million.
Dr Simon Gregson, from Imperial College London, who led the research, said: "Although we can't say for certain, fear of HIV and AIDS may have influenced this change in behaviour, with Zimbabwe's well educated population, good communications, and health service infrastructure, all combining to create this effect."
The researchers from Imperial College London and the Biomedical Research and Training Institute, Zimbabwe studied 9,454 people recruited from two household censuses, the first conducted between 1998 and 2000, and the second between 2001 and 2003.
They found that overall HIV prevalence declined from 23 percent to 20.5 percent. In men aged 17 to 54 it had declined from 19.5 percent to 18.2 percent, while in women aged 15 to 44, it declined from 25.9 percent to 22.3 percent.
Professor Geoffrey Garnett, from Imperial College London, and one of the researchers, said: "A key reason for this decline appears to be the reduction in the number of casual sexual relationships, although there was also a delay in the onset of sexual activity and increases in condom use prior to the time of the study may also have contributed".
The study found that among 17 to 19 year old men, only 27 percent had commenced sexual activity in the late r census, compared with 45 percent in the earlier one. For women aged 15 to 17, the percentage reporting sexual experience dropped from 21 percent to 9 percent. At the same time, the proportions of men and women reporting a recent casual sexual partner fell by 49 and 22 percent respectively.