The primary site of infection is the respiratory epithelium of the nasopharynx. Two to three days after invasion and replication in the respiratory epithelium and the regional lymph nodes, a primary viremia occurs with subsequent infection of the reticuloendothelial system. There is a second viremia that occurs five to seven days after the initial infection. P athologic changes in fatal measles usually represent the compound effect of viral and secondary bacterial infection.
Pneumonia is almost invariably present, most frequently interstitial.
More representative are changes of the uncomplicated viral diseases within the tonsillar, nasopharyngeal, and appendiceal tissue removed during the prodrome. These changes consist of round cell infiltration and the presence of multinucleated giant cells. S imultaneous with the onset of rash, measles-specific antibodies are detectable in serum. There is transient suppression of the tuberculin reaction, evident with the measles vaccine. In severe disease, the magnitude of depression of the total lymphocytes has been positively correlated with a lessened chance of recovery.