Naive adults, who never were vaccinated as a child or had measles as a child, are at a high degree of risk. From the link above, a 43 year old flight attendant from El Al airlines, is in a hospital in a coma from measles encephalitis. They may recover, or they may not....
Measles is not a trivial disease for naive adults, especially for women who may be pregnant or become pregnant
From the CDC website re: travellers and vaccination
"All travelers aged ≥12 months who do not have acceptable evidence of immunity to rubella (documented by ≥1 dose of rubella-containing vaccine on or after the first birthday, laboratory evidence of immunity, or birth before 1957) should be vaccinated with measles-mumps-rubella (MMR) vaccine. Before departure from the United States, infants aged 6–11 months should receive 1 dose of MMR vaccine (for measles protection), and children aged ≥12 months and adults should receive 2 doses of MMR vaccine ≥28 days apart on or after the first birthday. MMR vaccine is contraindicated during pregnancy. Pregnant women who do not have acceptable evidence of rubella immunity should not travel to countries where rubella is endemic or areas with known rubella outbreaks, especially during the first 20 weeks of pregnancy, and should be vaccinated immediately postpartum. Health care providers should also ensure that all women of childbearing age and recent immigrants are up-to-date on their immunization against rubella or have evidence of immunity to rubella, because these groups are at the highest risk for maternal-fetal transmission of rubella virus."
CDC website: www.cdc.gov/rubella