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view that SIV is a relatively benign infection for chimpanzees.
    Infected animals, they found, had between 10 and 16 times the mortality rate compared with uninfected chimps. Females carrying the virus were also less likely to produce offspring and, of those that did, the babies had much higher infant mortality rates compared with uninfected mothers and frequently picked up the infection through their mother’s breast milk, mirroring the breastfeeding risk seen amongst HIV-infected humans.
    The team also witnessed nine ‘new’ SIV infections amongst the animals they were following during the study and, by genetically sequencing the viruses involved, found that, just as in humans, most new cases arise throughinfection passing from individuals that have only recently been infected themselves. This is because when chimps first acquire the virus, they initially develop very high blood levels of the agent, making them much more likely to transmit SIV to other individuals during this early phase of the infection. The same is also true of humans infected with HIV. The team were also able to carry out post-mortems on some of the chimps that died during the study. One animal they examined showed chronic muscle wasting and shrinkage of the liver, multiple abscesses in the abdomen and depletion of the key CD4 immune cells targeted by the virus. Tissue samples obtained from other animals also showed similar parallels with human AIDS, including the loss of white blood cells from the spleen.
    So, far from being a walk in the Gombe National Park, SIV is a serious and often fatal infection for Africa’s chimpanzees. Apart from overturning an inaccurate dogma, this research also helps to highlight an important future avenue of research: chimps with SIV seem to develop the disease in the same way we do, but other primate species with their own forms of SIV do not. If scientists can track down which viral or hostfactors are responsible for the progression of the disease in some species but not others, it might be possible to uncover new ways to combat the virus in both humans and our next nearest relative.
    FACT BOX
    HIV biology
HIV/AIDS is, without doubt, the worst pandemic that humankind has ever faced. Researchers estimate that about 25 million people have already died of the disease and that about 35 million people are currently carrying it. In 2008, there were more than 7000 new HIV infections every day – one every 12 seconds.
Untreated, HIV slowly destroys the immune system by attacking key white blood cells that carry a marker on their surfaces known as CD4. When the supply of these cells is exhausted, which occurs after about 10 years on average, the infected person develops AIDS – and becomes vulnerable to a range oflethal infections caused by a host of different microbes ranging from viruses and bacteria to fungi and amoebae.
Scientists suspect that HIV ‘jumped’ into humans about 100 years ago, following close contact between a person – perhaps a hunter or a butcher preparing bush meat – and the blood of a chimp carrying SIV. Because humans and chimpanzees are so genetically alike, the SIV in the chimp is thought to have managed to infect the exposed human and then mutate within them to become the human AIDS virus. We know that HIV has jumped into humans in this way at least twice, because there are actually two types of HIV in circulation, HIV-1, which came from chimpanzee SIV, and a rarer form, largely confined to West Africa and called HIV-2, which came from a different primate species called a sooty mangabey.
When HIV infects an individual, it first has to break through the body’s main defence, the skin. For this reason, sex, needle sharing and the use of contaminated blood products are the main ways in which the virus is passed on.In the case of sexual transmission, researchers have discovered recently that the virus triggers the cells that make up the surface layers of the