Causes. The cause of rubella is a virus that's passed from person to person. It can spread when an infected person coughs or sneezes, or it can spread by direct contact with an infected person's respiratory secretions, such as mucus. It can also be transmitted from a pregnant woman to her unborn child via the bloodstream. A person with rubella is contagious from 10 days before the onset of the rash until about one or two weeks after the rash disappears. An infected person can spread the illness before the person realizes he or she has it.