Betsy Green, age 13, of North Brunswick, N.J., for her question:

WHAT IS THE FLU?

Influenza is the name of an infectious disease that is caused by the influenza virus and commonly called the flu.

We've all had the symptoms of the flu: chills, fever, headache, aches in the joints and general weakness.

Influenza is primarily a respiratory disease. The virus is inhaled medical men report, and it comes in contact with the cells of the upper air passages. The influenza virus penetrates the cells that line these passages and reproduces within them.

Many people develop immunity or resistance to influenza when the body produces substances called antibodies, and they are spared the agonies of winter attacks of the flu.