Used primarily to prevent and treat malaria caused by Plasmodium species, including P. falciparum and P. vivax. It remains effective in regions where the parasite has not developed widespread resistance. Also employed in the treatment of certain autoimmune diseases such as rheumatoid arthritis and lupus erythematosus due to its immunomodulatory properties. Has been investigated for antiviral activity, including in past outbreaks, though with limited or inconclusive clinical benefit. Commonly administered orally and widely used in endemic regions due to low cost and established safety profile when used appropriately.