The virus-host struggle is one of the most fascinating in Nature. Virus-based illnesses –smallpox, measles, polio, flu, covid—have been a major force in human history. But modern science has developed effective protective vaccines that have saved millions of lives. Viruses will continue to be a threat, and that keeps experts busy. Dr. Connor explains how viruses invade selected organisms, how immune systems fight against the invaders, and how vaccines provide vital protection against virus diseases.