Disclose is a method of treating alphavirus infections, particularly in humans, in which pentosan polysulfate is administered to an infected subject. Whilst not effecting the viral load in a subject, the pentosan polysulfate acts to reduce inflammation in tissues, such as the muscles, and in the joints of a subject. In addition, cartilage damage in the joints may be reduced. The reduction in inflammation and/or cartilage damage acts to reduce the severe pain experienced by subjects suffering from alphavirus infections, such as Ross River virus, chikungunya virus and Barmah Forest virus.