Plasmodium vivax infects only reticulocytes. Clinical symptoms from infection with Plasmodium vivax usually occurs 7 to 10 days after infections. For some patients, the symptoms of headache, photophobia, muscle aches, anorexia, nausea and sometimes vomiting can occur before the parasite can be detected in the bloodstream. The patients experience tertian fever cycles (48-hour cycles).
An initial untreated attack may last from 3 weeks to 2 months or longer. Eventually paroxysms become less severe and more irregular in frequency and then stop. In more than half of patients, relapses can occur after weeks, months, or up to 5 years or more. True disease relapse or recurrence, referring to reinvasion of the RBCs by liver merozoites (from hepatic hypnozoites) after complete clearing of the bloodstream by therapy or immunity, is seen only with P. vivax and P. ovale. Rarely, severe complications can occur, though coma, and sudden symptoms of cerebral involvement have been noted. Duffy blood group antigens serve as receptor sites for P. vivax, and so Duffy negative individuals are protected from P. vivax.