Tuberculosis is caused by the bacterium Mycobacterium tuberculosis, which grows in straight or branching chains and has a unique waxy cell wall containing mycolic acid, making it acid-fast and weakly gram-positive. Risk factors for tuberculosis include overcrowding, malnutrition, HIV/AIDS, other immunocompromised states, chronic diseases, malignancy, genetic susceptibility, surgery, injection drug use, smoking, working or living in high-risk settings, and being an adult woman, older, or younger than 4 years old.