Dear Dr. Donohue: My wife was having a long siege of severe coughing. After examining her and ordering a lung biopsy, the doctor diagnosed interstitial pneumonitis. He could not tell us how this came about, except that her immune system was out of whack and attacking lung tissue. Now she is doing well on oxygen and is taking prednisone, which is supposed to correct her problem.

With all the talk about AIDS, we wonder about that being involved. Also we worry about the prednisone causing infections, which we've been informed is possible because it affects the immune system. Is what she has a form of AIDS? Also, doesn't prednisone make her infection-prone?

Let's clear up the AIDS confusion first. Acquired immune deficiency syndrome involves a general breakdown of the whole immune system. Its cause is infection with a special virus - the human immunodeficiency virus. Your wife does not have that infection.

Your wife does have one of those mysterious illnesses in which the immune system attacks body tissue. That causes the inflammation and coughing. Such immune attacks occur spontaneously and for reasons shrouded in mystery.

One way to treat the immune problem and the consequent inflammation and cough is with prednisone, a cortisone drug. …