Question: what makes a virus contagious or not?

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  1. A virus is contagious or not (in a specific host, and a specific environment) because of the way it spreads between hosts. Contagious viruses are usually spread by some sort of physical contact or by the way the host leaves the virus lying around the environment. For instance, the flu is contagious because when you sneeze, or don’t wash your hands, you leave the virus in the environment for someone else to touch and then transmit to themselves. This is part of the reason why it’s so important that you wash your hands often!

    Non-contagious viruses and other diseases are usually spread by other methods, often what we call ‘vectors’. For instance, dengue fever is a tropical virus that has to be spread by mosquitoes. Without the mosquitoes, the virus won’t spread between hosts, so it’s classed as non-contagious.