At the end of World War I an epidemic swept over Europe and the United States, killing a tremendous number of people. The disease was influenza, or "flu." Influenza usually is not a dangerous disease. It is, however, rather tricky. People who have influenza often think that they are over it and get out of bed too soon. Then, a few days later, they get sick again and have to go back to bed.
A person with flu usually has chills and fever. He is likely to ache all over.
Scientists had a hard time finding out what causes influenza. Now they have found that it is caused by a virus. A person can have the disease again and again. But vaccines have been developed that help keep people from having it.
