Health officials in Japan say nearly 2,000 people have contracted rubella, or German measles, this year.
The infections are spreading from the Tokyo metropolitan area to other parts of the country.
The National Institute of Infectious Diseases in Tokyo announced on Tuesday that 154 new cases were reported in the week up to November 4th, bringing the total to 1,884.
Most of those infected are in the Tokyo metropolitan area, but cases were also found in central, western, and south-western parts of the country.
The number of men infected is 4.5 times greater than that for cases among women. Over 80 percent of the men are aged in their 30s to 50s.
Health regulations in place from 1977 to 1995 required only that junior high school girls be vaccinated for rubella, making men who are now between the ages of 30 and 60 vulnerable to infection.
Among infected women, those in their 20s form the largest proportion.
Rubella is a viral infection that causes fever and rashes. Doctors say an infection during pregnancy could result in the baby suffering visual, hearing or heart disorders.
The authorities are calling on women to have been vaccinated twice before becoming pregnant.
They say it's also important for family members of pregnant women or anyone in frequent contact with them to get vaccinated.