Bacteria like Streptococcus pneumoniae and Haemophilus influenzae are responsible for the disease pneumonia in humans which infect alveoli (air filled sacs) of the lungs. As a result of infection, the alveoli get filled with fluid leading to severe problem in respiration. A healthy person acquires the infection by inhaling the droplets/aerosols released by an infected person or even by sharing glasses and utensils with an infected person.
