A virus can enter the body in many ways besides food, fluids, and bites from insects or animals. The infectious secretions from viruses are passed on to others by coming in contact with them. They can be inhaled from airborne particles from coughs and sneezes. Viral particles may be on surfaces of toys, doorknobs, tables, or telephones. There are approximately 200 known viruses that cause disease in humans.
Viruses attack different parts of the body to produce different problems.
Some cause a sore throat, cough, runny nose, headache, or muscle ache. Others cause abdominal symptoms such as nausea, cramping, and diarrhea. Many viruses are hard to destroy without damaging or killing the living cells they infect, this is why drugs are not used to control them. Many viral diseases can be prevented by immunization. Colds, flu, and sore throats often recur in children.