Haemophilus influenzae, Streptococcus pneumoniae, and Moraxella catarrhalis are the most common bacterial pathogens in upper and lower respiratory tract infections. Bacterial pathogens adhere to mucous membranes and colonization occurs. In an otherwise healthy individual, the host’s immune system reacts to the invading bacteria, resulting in edema and swelling. If antimicrobial treatment does not destroy the invading organisms and successfully interrupt the progression of the infection, the patient may develop recurrent or chronic disease.