Illustration of a measles virus particle. The measles virus has only been found in humans, causing infection in the respiratory system. Measles is highly contagious, spread by coughing and sneezing, close personal contact or direct contact with secretions. A virus particle (known as a virion) is made up of a protein outer shell, a capsid, which protects the genetic information inside. Viruses are too small to be visible with optical microscopes so scanning and transmission electron microscopes are used to visualise them.