Mycobacterium tuberculosis is an aerobic, rod-shaped pathogenic bacterium that causes the highly infectious tuberculosis. M. tuberculosis has a unique, thick, waxy cell wall that makes it difficult to identify with Gram staining. The genome is 4 Mbp in size, with a proteome consisiting of over 3,900 genes. The pathogen transmits through aerosols and begins colonization in the lungs. Widely prevelant, an estimated one third of the world's population is believed to be infected. Tuberculosis is known to only infect humans. Other methods of detecting M. tuberculosis include the acid-fast stain, cell culture, and PCR.