Upper Respiratory Infection is an imprecise term referring to almost any infections disease process involving the upper airway, including nasal passages, pharynx, trachea, or main bronchi. The cause of an upper respiratory infection may be bacterial, fungal, or viral, and is rarely accurately understood. 
Although a Canadian report indicates that cannabis is commonly used by North Americans to cure symptoms of the flu and common cold,  upper respiratory infection may in fact be a health risk associated with smoking cannabis. Such infections may be in at least some cases related to smoking methods or contamination of marijuana with mold or toxic compounds.
The largest study of its kind in Australia recently focused on 268 long-term marijuana smokers. According to the chief investigator for the Commonwealth Department of Health National Drug Strategy,
We found nothing startling. We didn’t see any evidence of high psychological disturbance among the people. We see very little evidence of health problems except for respiratory problems. [David Reilly, drug and alcohol program manager, Northern Rivers Health Service] 
The early research of Tashkin et al. and Bloom et al. suggests that chronic cannabis smoking increases the prevalence of bronchitis and may cause an increased susceptibility to bronchogenic carcinoma in chronic cases. However, Hubert et al. point out, “There is still no conclusive evidence in man of clinically important pulmonary dysfunction produced by smoking marijuana.” 
Related sections: Contaminants, Immune Responses, Respiratory Diseases, Smoking Methods, Tolerance.
 Taber’s Cyclopedic Medical Dictionary. Philadelphia: F.A. Davis Company, 1987
 Canadian Government Commission of Inquiry into the Non-medical Use of Drugs. Ottowa: Information Canada, 1972
 Lamont, “Study Shows Marijuana Users as Healthy as the General Population.” Sydney Morning Hearald, February 18, 1997
 “The health and psychological consequences of cannabis use.” Chapter 6, National Drug Strategy Monograph, No. 25, Australia