Some people refuse to believe that HIV causes AIDS, despite all the evidence (Image: Spencer Platt / Getty) A patient holds her HIV/AIDS antiretroviral (ARV) drugs (Image: Sipa Press / Rex Features) Peter Duesberg, a superstar scientist amongst AIDS denalists (Image: Wikipedia Commons) Former South African president Thabo Mbeki was criticised for being an AIDS denialist Seth Kalichman who went undercover to understand the minds and ideals of AIDS denialists (Image: The University of Connecticut 2009)

ON 27 December 2008, a well-heeled 52-year-old woman died in a Los Angeles hospital. Her death certificate describes a body riddled with opportunistic infections typical of the late stages of AIDS. Christine Maggiore had tested HIV positive 16 years earlier, but she had shunned ART, the antiretroviral therapy that stops HIV replicating and prevents AIDS.

This was not the first time a death in Maggiore’s family had made headlines: three years earlier her 3-year-old daughter Eliza Jane had died. The autopsy described a chronically ill little girl who was underweight, under-height, and had encephalitis and pneumonia – all AIDS-related. When pregnant, Maggiore had again rejected ART and she had breastfed Eliza Jane, another way of transmitting the virus.

Why, in 21st-century California, would a middle-class woman and her young daughter die like this when there is tried-and-tested treatment for their illness? The answer lies in a bizarre medical conspiracy theory that says AIDS is not caused by HIV infection (see Five myths about HIV and AIDS).

It is tempting to dismiss the so-called AIDS denialism movement out of hand, but it has a strong internet presence, with a plethora of websites and blogs that …