There are three major types of tubercle bacilli that affect humans. The human type ( Mycobacterium tuberculosis ), first identified in 1882 by Robert Koch, is spread by people themselves. It is the most common one. The bovine type ( M. bovis ) is spread by infected cattle but is no longer a threat in areas where pasteurization of milk and the health of cattle are strictly supervised. The avian type ( M. avis ) is carried by infected birds but can occur in humans. The tubercle bacillus can live for a considerable period of time in air or dust. The most common means of acquiring the disease is by inhalation of respiratory droplets.