In 1991 the incidence of TB in Canada was 7.5 cases per 100,000. Most of these cases occurred in immigrants from countries of high prevalence and in aboriginal persons.
Infection occurs when airborne bacteria settle in the lungs.
There they multiply and are carried by the lymphatic system and blood to other parts of the body. After 2-10 weeks the infection is arrested, bacteria is dormant but may progress to active disease at some later stage in life. A person infected with M. tuberculosis is not infectious to others, in order to be infectious, active disease must be present. The two major reasons for treatment failure and subsequent development of drug resistance are non-compliance and malprescribing. Expert advice should be sought if the smear or culture is positive or the patient is symptomatic after three months of therapy.