Measles is a highly contagious viral infection, and certain groups are more at risk than others. Primarily, unvaccinated individuals, including young children, are the most vulnerable. Infants under 12 months are particularly at risk because they are too young to receive the measles, mumps, and rubella (MMR) vaccine. Moreover, individuals with weakened immune systems, such as those undergoing chemotherapy or living with HIV, face increased susceptibility to severe complications from measles. **Unvaccinated Populations** The most significant risk factor for contracting measles is lack of vaccination. Those who have not received the MMR vaccine are at a higher risk of infection, especially when exposed in high-density public places like airports. This is why public health alerts, such as the recent one at Chicago O'Hare, are crucial for informing potential exposure areas. Vaccination not only protects the individual but also contributes to herd immunity, reducing the spread of the virus. **Infants and Young Children** Infants under 12 months old are at high risk because they are too young to be vaccinated. As a result, they rely on the immunity of those around them. Young children who have not yet received both doses of the MMR vaccine are also at elevated risk. The first dose is typically given between 12 to 15 months of age, with a second dose between 4 to 6 years old. Ensuring timely vaccination is key to protecting this vulnerable group. **Individuals with Compromised Immune Systems** Those with compromised immune systems, such as individuals undergoing chemotherapy, living with HIV, or taking immunosuppressive medications, are more susceptible to measles and its complications. Their bodies are less equipped to fight off infections, making vaccinations and avoiding exposure critical. **Travelers and International Visitors** Travelers, particularly those coming from or going to regions where measles is more prevalent, are at increased risk. International visitors who may not be vaccinated or may come from areas with lower vaccination rates can inadvertently contribute to outbreaks in the U.S., making it important for travelers to ensure their vaccinations are up to date. In the context of public health alerts like the one at Chicago O'Hare, understanding who is most at risk can help in making informed decisions to protect oneself and the community. The alert underscores the importance of vaccination, awareness, and timely action in preventing the spread of measles.

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