The common cold is caused by several groups of viruses including rhinoviruses, coronaviruses and respiratory syncytial virus. Colds are the leading cause of doctor's visits and the most common reason for absences from work and school. Research suggests that routine vitamin C supplementation may modestly reduce a cold's duration and severity for the general population. While therapeutic trials of the vitamin for treatment of the common cold show no significant changes in the risk of catching a cold in the general population, statistics indicate that its anti-oxidative properties halve the risk of catching a cold in people exposed to extreme physical stress. In brief, it is unclear to what extent an increased vitamin C intake before or during a cold may be helpful.