Rubella is an infection caused by the rubella virus, which is usually transmitted by droplets from the nose or throat. Congenital rubella syndrome (CRS) is an important cause of severe birth defects. Even though it is a mild childhood illness CRS causes many birth defects. Deafness is the most common, but CRS can also cause defects in the eyes, heart, and brain. When a woman is infected with the rubella virus early in pregnancy, she has a 90% chance of passing the virus on to her fetus. This can cause the death of the fetus, or it may cause CRS.