A varying flora of indigenous commensal organisms (including Pasteurella multocida, Bordetella bronchiseptica, streptococci, staphylococci, pseudomonads, and coliform bacteria) normally reside in the nasal passages, nasopharynx, and upper trachea, and at least intermittently in the lungs of dogs and cats, without causing clinical signs. Opportunistic infections by these bacteria may occur when respiratory defense mechanisms are compromised by infection with a primary pathogen (eg, distemper , parainfluenza virus , or canine type 2 adenovirus in dogs, and rhinotracheitis virus or calicivirus in cats), other insults (eg, inhalation of smoke or noxious gases), or diseases such as congestive heart failure and pulmonary neoplasia .