Chickenpox is an infectious disease caused by the varicella-zoster virus. The virus is spread through droplets. It is usually harmless in children with a mild fever, tiredness and itchy rash (red spots or blisters). Serious complications in childhood are rare. In newborns, adults with weakened immune systems and in pregnant women, the varicella viruses can cause shingles (herpes zoster), which can lead to severe and long-term pain.