Doctors in Thailand and Japan have used HIV medications to treat patients infected with the novel coronavirus with apparent success. But why did they think to test drugs made to combat one pathogen on a seemingly completely different disease?HIV, the virus that causes AIDS, hijacks white blood cells to replicate itself and eventually impairs the host's immune system. Once seen as incurable, HIV can now be brought under control by antiretroviral therapy -- mixtures of antiviral drugs that interfere with the virus's ability to reproduce, letting patients live normal lives.These include lopinavir and ritonavir, the medications used in the experimental coronavirus treatments. Both belong to a class of drugs known as protease inhibitors.