In humans, viruses can spread in many ways. Four fluids that can be used by viruses to spread are:
- Air: examples of viruses that spread through air are those that cause influenza and the common cold. When people with these illnesses sneeze or cough the viruses are spread to others.
- Water: viruses that cause illnesses like gastroenteritis are spread from one person to another by infected water.
- Blood: a large number of viruses are spread through blood. Diseases like West Nile Fever can be spread through vectors that suck blood. As the viruses can survive in the vector's body also they are spread when the same animal bites a person who is sick and then bites a healthy person. Some viruses like HIV cannot survive in a vector's body. They are spread when a healthy person's blood comes in contact with blood from someone who is sick through infected needles, or during blood transfusions.
- Sexual fluids: viruses which include HIV are spread though sexual fluids like semen and vaginal fluids. This usually happens during sexual intercourse.