Facts About Tuberculosis and Disease

The causative agent of tuberculosis (TB) is a bacterium. Humans become infected by inhaling air containing particles expelled by an infectious person. Within 2 to 10 weeks after infection, the immune system usually intervenes, halting the multiplication of bacteria and preventing further spread.

The tuberculin test is used to identify persons who have been infected with TB bacteria. Most infected people have a positive reaction within 2 to 10 weeks after infection. Persons who are infected but who do not have TB disease are not infectious to others. TB infection in a person who does not have TB disease is not considered a case of TB.

TB infection progresses to a disease when the TB bacteria overcome the defenses of the immune system and begin to multiply. This can happen soon after exposure or it can be delayed by many years or even decades after infection. In the United States approximately 10% of persons infected with TB bacteria will develop TB disease at some point in the future; the remaining 90% will not progress to TB disease.

Screening for TB infection or disease should always be carried out in consultation with the local health department, which is primarily responsible for control of communicable diseases in the community. Clinicians should identify patients who are in a high-risk category, and they should tuberculin test these persons. Persons with positive reactions should be evaluated for TB disease and, if disease is ruled out, considered for preventive therapy.

When a person is identified with TB disease, a contact investigation should be undertaken beginning with close contacts -- persons who spend the most time with the person who has infectious TB. Also, a high priority should be given to contacts who are children or who are HIV infected, because they are more susceptible. If the TB infection level of close contacts of the index case is higher than expected for the community, then the investigation should be expanded to other contacts.