Three things to know:
1. The outbreak has sickened at least 21 people, 20 of whom are children. Health officials are also investigating four more suspected cases.
2. Of those sickened, 18 were not vaccinated against the disease.
3. The outbreak may continue to spread, as sickened individuals visited several public places, including schools, restaurants and the Portland (Ore.) International Airport while contagious.
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