Abstract: A water extract of homogenized Cryptosiphonia woodii is useful for the treatment of herpes simplex viral infections. The active agent in the extract is a polysaccharide containing glucose and galactose.Herpes infections may be treated in subjects, e.g. human patients, by administering to the subject an effective amount of an aqueous extract of C. woodii, particularly a water extract. This treatment method is effective for treating subjects both prior to and subsequent to infection. It may involve topical application to alleviate symptoms associated with herpes infections or desirably may be systemic, e.g. by oral administration, to eradicate the virus and thereby prevent symptom recurrence.