The Centers for Disease Control and Prevention has recorded a third measles death in the United States this year. The agency added the fatality to its federal count as the national caseload reached more than 4,400 cases. The combination of three deaths and a large number of infections gives the current outbreak a serious human cost. Measles is highly contagious, and complications can be severe for people who lack immunity or have weakened immune systems. The federal update did not provide the victims' identities, locations or other details about the outbreak's reach. Public-health containment generally depends on high vaccination coverage, prompt identification of cases, isolation of infected people and tracing of contacts. Gaps in those steps can allow transmission to continue.