Measles (Rubeola)

Measles is a highly contagious viral illness caused by the rubeola virus, a single-stranded RNA paramyxovirus that is transmitted exclusively between humans through respiratory droplets and airborne particles. Measles is one of the most contagious infectious diseases known, with a basic reproduction number (R0) of 12-18, meaning one infected person can transmit the virus to 12-18 susceptible individuals. The virus enters the body through the respiratory epithelium or conjunctival mucosa and initially replicates in local lymphoid tissue. Within 2-3 days, a primary viremia distributes the virus to the reticuloendothelial system (liver, spleen, lymph nodes), where it undergoes massive amplification. A secondary viremia then disseminates the virus to the skin, respiratory tract, and other organs, producing the characteristic clinical syndrome. The pathognomonic feature of measles is Koplik spots -- small, white-blue papules on an erythematous base that appear on the buccal mucosa opposite the molars 2-3 days before the rash onset. These represent focal areas of epithelial necrosis and mononuclear cell infiltration. The classic measles prodrome includes the 3 Cs: cough (from tracheobronchitis), coryza (runny nose from upper respiratory inflammation), and...