It is a virus that attacks the immune system, which is why it's called immunodeficiency virus, and so it causes the immune system to become weak, and when it becomes weak, people become unwell.

When a person first catches HIV, very often, they may not have any symptoms at all.

And many people can live for many years after catching HIV with no symptoms, but some people develop symptoms very soon after they catch HIV, which are basically the symptoms of a severe viral infection.

So they may develop flu-like symptoms—cough, fever, sweats, enlarged lymph nodes, and they may feel very unwell.

When a person first catches the HIV virus, as I said before, they may not develop any symptoms at all.

But after they've had HIV infection for a while, sometimes for several years, as the immune system becomes weaker, the body stops being able to fight off simple infections that live normally in the environment.

And when people become unwell with these infections that live commonly in the environment, but that people with normal immune systems are able to fight off themselves, that is said to be called AIDS.

And AIDS basically refers to a group of illnesses that people with normal immune systems rarely get, and these illnesses may affect all the systems in their body.

They may affect the brain, they may affect the skin, they may affect the lungs, and they may affect other internal organs.

And when people have these infections, they are usually extremely unwell, and without treatment, most of them will die.

HIV is treated now very effectively with medicines called antiretrovirals.

Antiretrovirals attack the virus and stop it from affecting people's immune systems.

And so now, with effective treatment, people don't develop what used to be called AIDS, so people's immune systems do not deteriorate, and people do not become unwell.

But most importantly, people on effective treatment for HIV can't transmit the virus to other people.

So they are uninfectious.

And that's probably one of the most important things that people should know about HIV.

So a person on effective treatment is basically very similar to somebody who doesn't have HIV at all in that they can't transmit the virus to other people.

They can't transmit it to their sexual partners, but neither can women who are pregnant transmit it to their babies, which is something that used to happen when treatment was not available.

We now know also that the life expectancy of a person who has HIV and who is on effective treatment will be the same and sometimes even longer than the life expectancy of somebody without HIV, so treatment has been very successful in this area.