It usually takes lengthy contact with someone with active TB before a person can become infected. On average, people have a 50 percent chance of becoming infected with M. tuberculosis if they spend eight hours a day for six months or 24 hours a day for two months working or living with someone with active TB. However, people with TB who have been treated with appropriate drugs for at least two weeks are no longer contagious and do not spread the germ to others.
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