Which disease is NOT spread by ticks?

Conjunctivitis is primarily an inflammation of the conjunctiva, often caused by infections, allergens, or irritants. Unlike Lyme disease, Rocky Mountain spotted fever, and human ehrlichiosis, conjunctivitis is not transmitted by ticks.

Tick-borne diseases are specifically caused by pathogens that ticks can transmit through their bites. Lyme disease is caused by the bacterium Borrelia burgdorferi, Rocky Mountain spotted fever is caused by Rickettsia rickettsii, and human ehrlichiosis is caused by bacteria in the genus Ehrlichia, which also rely on ticks for transmission to humans.

In summary, the distinguishing factor here is that conjunctivitis is not associated with tick bites or transmission, marking it as the correct choice in identifying a disease that is not spread by ticks.