InfluenzaInfluenza is caused by a virus. "It's a very bad cold that can turn into pneumonia and be dangerous, even fatal," McCarthy says. "The very young, the very old, and those with health problems like asthma are most at risk of complications. It's spread through coughs, sneezes, and contact with the saliva and other secretions of people with the illness. There are medications that are sometimes used, but like the medications for chickenpox, all they can do (if anything) is make the illness milder and shorter."