Humans become infected by M.tuberculosis commonly by

M.tuberculosis is mainly transmitted by direct inhalation of aerosolised bacilli contained in the droplet nuclei of expectorated sputum. Generated while coughing. sneezing,or speaking of infected patients. There may be as many as 3000 infectious nuclei per cough. The tiny droplets may remain suspended in the air for several hours and are easily inhaled. Ref: Textbook of microbiology; Ananthanarayan and paniker's; 10th edition; Pg: 355