Coloured scanning electron micrograph (SEM) of drug-resistant Mycobacterium tuberculosis bacteria (MDR-TB and XDR-TB strain). M. tuberculosis is the primary causative agent of human tuberculosis (TB), an ancient disease which still causes more adult deaths than any other single infectious disease. Contaminated respiratory secretions transmit the infection between humans. Drug-resistant strains, MDR-TB and XDR-TB strains have emerged, especially in developing countries. MDR-TB (Multidrug Resistant TB) are strains of M. tuberculosis that are resistant to at least the two first-line TB drugs, isoniazid and rifampicin. XDR-TB, or Extensive Drug Resistant TB (also known as Extreme Drug Resistance) is a MDR-TB strain that is also resistant to three or more of the six classes of second-line drugs. Drug-resistant TB is a public health issue in many developing countries, as treatment is longer and requires more expensive drugs. XDR-TB poses a severe public health crisis, especially in populations where HIV is prevalent and where there are few healthcare resources. Magnification: x2,400 when shortest axis printed at 25 millimetres.