A chemical found in oregano has been found to have some effects on viral growth in laboratory settings and against some animal viruses. There is no evidence of any clear impact on viruses among humans.
A 2003 study concluded that the chemical, carvacrol, demonstrated anti-viral properties against a virus affecting mice. A later study, in 2011, found Mexican oregano oil had varying effects against different viruses.
A 2018 study found carvacrol had varying positive effects on antimicrobial, antioxidant and anticancer activities, but authors stated that the lack of human trials "largely impedes any conclusions of clinical relevance."