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Input: By Vanessa BarfordBBC News, Washington DC How common is it? More than 1.2 million people in the US are living with HIV - the virus that targets the body's immune system and can lead to Aids. Almost one in eight of them are unaware they are affected, according to the Centers for Disease Control and Prevention (CDC). The World Health Organization (WHO) puts the global figure at 35 million - with only 50% of people aware they have the virus. Sub-Saharan Africa is particularly badly affected, with nearly one in every 20 adults living with HIV. Nearly 71% of people who have contracted HIV live in Africa. About 107,800 people in the UK were living with the condition in 2013, according to the UK government. A quarter are thought to be oblivious. Are infection rates rising or falling? Globally, the annual number of new HIV infections fell by a third between 2001 and 2013, from 3.4 million to 2.3 million, according to a UN report. But they aren't falling everywhere. New infections in the US have remained at about 50,000 per year for the last decade - compared with 130,000 at the height of the 1980s epidemic. Gay, bisexual and other men who have sex with men account for 63% of all new US infections. In London, one in every eight men who have sex with men is HIV-positive, according to Public Health England. African Americans are also disproportionately affected. "African Americans only make up 12% of the US population, but 46% of new diagnoses of HIV infection," says Dr Anthony Fauci, director of the National Institute of Infectious Diseases in the US. There were about two million new HIV infections around the world in 2014 - 220,000 of which were among children. Most live in sub-Saharan Africa and were infected via their HIV-positive mothers during pregnancy, childbirth or breastfeeding, according to the UN. In 2015, Cuba was the first country declared to eliminate mother-to-child transmissions. How does HIV affect your life expectancy? Today, a 20-year-old who is newly diagnosed and receives combination anti-HIV drugs can expect to live 50 more years. That compares to a prognosis of months or years back in the 1980s. Much of the improvement is down to the discovery of antiretroviral therapy (ART), which prevents HIV from multiplying and reduces the amount of the virus in the body. "If people take a combination of three or more ART drugs, and stick to their medication regimen, it can completely change their outlook," says Fauci. At the end of 2014, 14.9 million people were receiving ART - 40% of the total number living with HIV. But 1.5 million people still died from HIV-related complications in 2013. About 39 million people have died from HIV since the mid-1980s. How risky is unprotected sex? Sheen says he had unprotected sex with two partners "under the care of [his] doctor". How risky this is depends on a number of factors. HIV is transmitted through the exchange of a variety of body fluids such as blood, semen, vaginal fluids and breast milk. Unprotected sexual intercourse is a common way of spreading the disease. Sheen's physician, Dr Robert Huizenga, told the Today show there's a very small chance the actor could pass the virus on to sexual partners because it's now "undetectable" in the actor's blood. The rate of HIV infection for negative partners is 96% lower if the positive partner is on treatment, an international study suggests. But the CDC says less than 40% of Americans with HIV are on antiretroviral treatment - and only 30% have achieved viral suppression, where the HIV becomes "undetectable". The amount of HIV in the blood also rises again, as does the risk of transmission, when people stop talking their medicine. A new type of drug treatment - Pre-Exposure Prophylaxis (PrEP) - can offer protection against HIV infection during sex but isn't widely available. When Sheen referred to partners under his doctor's care, it's possible he meant they were on this drug. Is spreading HIV a crime? People with HIV may be prosecuted for intentionally or recklessly infecting another person. In the US, 67 laws explicitly focused on people living with HIV had been enacted in 33 states by 2011. They cover not disclosing the virus to sexual partners - even if the transmission risk is minimal or nonexistent - donating HIV-infected organs and spitting HIV-infected bodily fluids. However, there have been calls to re-examine specific state laws in light of studies showing that ART reduces the risk of HIV transmission. Other countries, such as the UK, Finland and New Zealand, charge people under existing laws, with crimes such as grievous bodily harm, manslaughter and assault. The UN urges governments to limit criminalisation to cases where people have intentionally transmitted the virus. It says general - and not HIV-specific - laws should be used in these cases.

Are we winning the fight against HIV?