Ben Miller, age 15, of Rutland, Vt., for his question:

HOW DOES INFLUENZA SPREAD?

Influenza is an infectious disease caused by the influenza virus. We all know the unpleasant symptoms: chills, fever, headache, aches in the joints and weakness.

Influenza is mainly a respiratory disease. The virus is inhaled and comes in contact with cells of the upper air passages. It penetrates the cells that line these passages and reproduces within them. Influenza may spread deep within the lungs and to other parts of the body.

The virus may also be carried away in exhaled air and infect other people.

People develop immunity or resistance to influenza when the body produces substances called antibodies, which can attach themselves to influenza viruses and prevent them from infecting cells.

Health authorities try to prevent the spread of influenza by means of vaccination.