Four necessary conditions for HIV transmission: 1. Exclusion: there must be an infected person to exclude the virus; 2. Survival: there must be suitable conditions for the virus to survive, because it is weak to external resistance. Sensitive to heat, 56 degrees for 30 minutes can make HIV in vitro to human T-lymphocytes lose infectivity, but can not completely inactivate the HIV virus in the serum, 100 degrees for 20 minutes can be completely inactivated HIV. Can be inactivated by 75% alcohol, 0.2% hypochlorous acid and bleaching powder. 0.1% formaldehyde, ultraviolet light and r-rays cannot inactivate HIV; 3. Sufficient amount: to meet a certain amount before causing infection, such as blood visible to the naked eye; 4. Invasion: to invade the pathway, such as broken skin and mucous membranes.