Rubella virus infects its host via the respiratory tract (nasopharynx), replicates within host cells, is released into the blood stream (viraemia), and continues to replicate. The initial immune response is marked by the appearance of specific IgM antibodies that rise in titre over 2 to 3 weeks as the rash is fading.
In infected pregnant woman, rubella virus crosses the placenta to infect the embryo. Its greatest effect is on cells rapidly dividing during early gestation. Specific effects vary with time of fetal infection and are likely to be severe if the fetus is infected within the first 12 weeks.