Causes
Rubella is caused by a virus. It's spread in the same way as a cold,
by tiny droplets produced from the nose and mouth. It's not as contagious as
a cold or the measles, but it has a long incubation period (the time between
catching the virus and getting sick) that varies from 12 days to 23 days. People
who catch the virus are infectious (can spread it to others) for a full week
before symptoms appear.
Because rubella is more rare these days, an increasing number of adults have
never been infected with the disease and therefore aren't immune unless they
have been vaccinated. It's estimated that about 15% of women of childbearing
age are vulnerable to rubella.