Convalescent

Pertussis has three stages, and knowing them determines management. The catarrhal stage lasts one to two weeks with nonspecific coryza, mild cough, and low-grade fever, and is the most contagious period and the only window in which antibiotics alter the course. The paroxysmal stage lasts two to six weeks with severe coughing fits ending in an inspiratory whoop and post-tussive vomiting, along with marked lymphocytosis driven by pertussis toxin's inhibition of lymphocyte trafficking; by this point macrolides no longer shorten illness but are still given to reduce transmission. The convalescent stage may last months, giving the illness its name of the hundred-day cough, and residual cough may be triggered by subsequent viral infections. The term is used more broadly for the recovery phase of any illness, and 'convalescent serology' means a paired antibody titre drawn two to four weeks after the acute sample, with a fourfold rise taken as diagnostic; convalescent plasma is the transfer of antibody-rich plasma from recovered donors.