Q: Why is it said that the HIV virus dies once it is outside the body? All viruses become inactive once outside the body - is that not so, and moreover, as far as I know a virus never dies and can always get into its reproduction mode once inside the host cell. So, in this regard I want to know that if suppose I put a drop of HIV infected blood on a slide, keep it exposed for some time, say a couple of hours, and then if I bring an open wound in contact with the dried blood, will the person get infected with HIV?

A:HIV is a relatively fragile virus and is susceptible to drying, which means that it dies easily when exposed to environmental conditions outside the body. On the other hand, Hepatitis B virus is comparatively more hardy virus capable of tolerating more adverse conditions. Also, the blood required to transmit Hepatitis B is one thousandth that required for transmitting HIV. Though usually, the HIV virus dies off within 30 minutes outside the body, some experiments have shown that it can survive for 3 days outside the body, if the initial viral load was high, and not directly exposed to the environment. The risk of HIV infection from a needle suspected to be contaminated with HIV is between 0.2% and 0.5%, estimated on the basis of a number of studies of occupational exposure to HIV in the hospital setting. The risk is increased with higher viral load, which is related to the amount of blood introduced. There are many other variables (apart from volume of blood and viral load) that affect the transmission of HIV - the size of the needle, the depth of penetration and whether or not blood was injected are important considerations. It is difficult to answer the theoretical question without more concrete details.