Ask the Doctor: Viruses Spread Differently
DEAR DR. DONOHUE:
With colds and the flu upon us, I would like to know: 1. How long are germs contagious? If someone coughs and the droplets land on a phone, how long will the germs survive? 2. Do you have to sneeze to spread germs, or can germs spread simply by talking? 3. If I hold a baby who has a cold, is there any way to prevent the spread of germs? 4. How many days before symptoms is someone contagious, and how long does he or she remain contagious with a cold or sore throat? - M.E.
Every germ - virus or bacterium - has different properties, and the number of germs that cause colds and sore throats is large. So I have used the most common germs as the prototypes for my answer.
Cold viruses are spread mostly by the fingers and hands. Hand-washing, therefore, is the best way to prevent spread and infection. Droplets from coughs and sneezes also can spread cold viruses, but the range of spread is only 3 feet. If you are farther than 3 feet away from people, you won’t get their cold viruses. Talking doesn’t propel viruses to any great extent. Inanimate objects (e.g., telephones) don’t support viral existence for very long. If a person with a cold uses a phone, the next user might pick up the cold virus, but not the following person. If a baby has a cold, don’t pick the child up if he or she is not yours.
An infected person can spread the cold virus from one day before symptoms appear to as long as two weeks after their appearance. The virus population is at its maximum on days three and four of the cold, and that’s when a person is most contagious.
Viruses and bacteria cause sore throats. For viral sore throats, what I said about colds is a rough guide. For strep throat, a person no longer spreads the germ after one day of treatment. If untreated, people can be infectious for 10 to 21 days. Large respiratory droplets from the nose and throat transmit the bacterium.
Flu is spread in airborne droplets from a cough or sneeze. An infected person can spread the virus within 24 hours of developing symptoms, and peak shedding occurs in two days and lasts five to 10 days. Flu viruses persist for hours on inanimate objects, but such objects are not a great source for spread.