So far this year, 107 people have contracted the measles across the United States, according to the CDC. That includes Missouri and Illinois.
There were 118 cases in all of 2017.
Missouri and Illinois require schoolchildren to get their measles vaccines, but they can be exempted for a medical or religious reason.
Doctors warn that not getting vaccinated could have serious consequences.
Measles is usually passed through coughing and sneezing.
At this time the CDC does not consider this an outbreak. This is their statement on why that is the case:
"A confirmed measles outbreak as defined by the World Health Organization is three or more confirmed measles cases (at least two of which should be laboratory-confirmed; IgM positive) in a health facility/district/block (approximate catchment population of 100 000) in a month (an example of a measles outbreak would be the one reported last year in Minnesota that we described here). The 107 cases reported to CDC have been since January 2018 across 21 states, but they are not one unified outbreak. CDC works with local and state health departments throughout the year on any number of infectious diseases, including measles cases."