"To fight the virus" means taking actions and measures to prevent the spread of a harmful disease or infection (virus) and to protect people's health and wellbeing. Full definition
No vaccine is available for this form of equine herpes, but there are vaccines that can boost the horse's immune system to help fight the virus. (sciencedaily.com)
A key immune protein wasn't produced and thus was not fighting the virus. (sciencemag.org)
Usually, the body fights the viruses and bacteria, but if the immune system fails to do this, a tumor will form. (ahealthblog.com)