In humans, influenza’s effects are much more severe and last longer than those of the common cold. Recovery takes about one to two weeks. Influenza, however, can be deadly, especially for the weak, old or chronically ill. The flu can worsen chronic health problems. People with emphysema, chronic bronchitis or asthma may experience shortness of breath while they have the flu, and influenza may cause worsening of coronary heart disease or congestive heart failure. Smoking is another risk factor associated with more serious disease and increased mortality from influenza.
Symptoms of influenza can start quite suddenly one to two days after infection. Usually the first symptoms are chills or a chilly sensation, but fever is also common early in the infection, with body temperatures as high as 39