Who Catches Cold, and Why? Q. If 100 people are exposed to my cold, how many will catch it, and why some and not others? A. Transmission rates can range from 50 to 80 percent, estimates Dr. Elizabeth Jacobson, at the Iris Cantor Women's Health Center at NewYork-Presbyterian/Weill Cornell Medical Center. As for why some are spared, there are many possibilities. "There are hundreds of strains of cold viruses, and they have different rates of infectivity," said Dr. Jacobson, an internist and allergist. Previous exposure to the virus at hand should lead to immunity, but immunity can wane over time, and viruses mutate as well. Resistance can be compromised by an array of factors, from sleep deprivation to underlying respiratory diseases, she said. Immunodeficiency, whether caused by medication or disease, can also predispose a person to catch cold. People who wash their hands less often or touch their eyes and nose more often are more likely to catch a cold, as are those who spend more time with an infected person. It is possible to have a cold without noticeable symptoms, which vary from strain to strain and person to person, Dr. Jacobson said. Communicability also depends on the amount of the virus in the cold carrier's secretions. It is higher earlier in the cold and begins to drop after three or four days. Also, air temperature and humidity can affect how long cold viruses can live on inanimate surfaces, she said. Readers may submit questions by mail to Question, Science Times, The New York Times, 620 Eighth Avenue, New York, N.Y. 10018, or by e-mail to question@nytimes.com.