Overview. Viral infections are the most common cause of illness in children. They are highly contagious. Most viruses cause symptoms over a short period of time and produce mild illness.Some viruses can cause significant disease, especially in those who have chronic or debilitating conditions.he most common viral causes of meningo-encephalitis are enteroviruses, arboviruses (the most recognized arbovirus is the West Nile virus), and the herpes virus. Herpes can cause a devastating disease, usually presenting with seizures. In newborns, it can lead to permanent brain damage.