Varicella, commonly known as chickenpox, is a highly contagious viral infection caused by the varicella-zoster virus. It primarily affects children but can also occur in adults who have not been vaccinated or previously infected. Symptoms include an itchy rash, fever, and fatigue, typically appearing 10 to 21 days after exposure. The rash starts as small red spots that develop into fluid-filled blisters, eventually crusting over. Vaccination is the most effective way to prevent varicella, significantly reducing the incidence and severity of the disease. In some cases, the virus can reactivate later in life, causing shingles.