Recurrent infections constitute a major challenge to primary care physicians. Primary immunodeficiency or other alterations of the host defense system are extremely rare. In the case of recurrent respiratory infections, particularly of the lungs, a diagnosis will result much more often from ordering a sweat chloride test than a serum immunoglobulin level. RECURRENT RESPIRATORY INFECTIONS The major reason for referral to our clinic for investigation of the immune system is a complaint of too many upper respiratory tract infections. Each respiratory infection in a young child means loss of sleep for child and parents, expenditure of time at the physician's office, and possible loss of income in addition to the expense of medication and office visits. The child is often irritable and his or her misery clouds the home atmosphere. With repeated episodes of this sort each year, it is small wonder that parents want relief. The average number of upper respiratory tract infections experienced by healthy children is approximately 9 to 10 per year. At least 100 viruses can cause the common cold. Accordingly, even when a child has a normal immune system establishing immunity to each virus or virus subtype, it can take many years for a broad enough immunity to develop to diminish the frequency of infection due to newly encountered respiratory viruses.