HIV is a virus that kills by crippling our defences against other infections. It sends our immune system into a creeping decline. Germs that were once easy to fight off now become debilitating and lethal threats. A simple cold can kill. Tumours start to grow.

This is AIDS. It was formally described in 1981 and now, over 30 years later, we’re finally starting to understand why it happens.

HIV can infect many different types of white blood cell, but chief among them are the CD4 T-cells. These are the bugle-players of the immune system—they mobilise other immune cells, which actively kill viruses and other invaders. HIV prevents these troops from entering the fray, because it slowly destroys the CD4 T-cells.

Only a minority