A 22-year-old homeless person with a known drug abuse problem and multiple opportunistic infections has a positive PPD (purified protein) test. Which of the following is the most common way this infection is acquired?

The respiratory tract is a common portal of entry to such airborne organisms as M. tuberculosis. This is why respiratory precautions must be taken when patients are harboring viable M. tuberculosis. The GI tract is usually infected from ingestion of contaminated food or water (Shigella, Salmonella, and Campylobacter) or by an alteration of the normal microbial flora such as with C. difficile disease. The genital tract may become infected either by sexual contact or by alteration of the genital environment, as often occurs with yeast infections. Several bacteria such as N. gonorrhoeae, Chlamydia, and T. pallidum are transmitted by direct sexual contact with infected partners.