The Wisconsin Department of Health Services said the outbreak remains concentrated in the state's southwest, where Grant County has recorded 153 cases, Lafayette County 122, Iowa County 97, Vernon County 14, Sauk County 10, Rock County five and Richland County one.

The new case counts follow a grim milestone last week: Grant County reported the death of an infant due to the outbreak, the first fatality linked to the current spread of the virus.

Measles is among the most contagious infectious diseases known to science, capable of lingering in the air for up to two hours after an infected person has left a room. It can cause severe complications, particularly in infants, pregnant people and those with weakened immune systems, including pneumonia, brain swelling and death.

The virus was declared eliminated in the United States in 2000, but declining vaccination rates in some communities have allowed outbreaks to resurge. Public health experts say a community needs roughly 95% of its population vaccinated with two doses of the measles-mumps-rubella vaccine to prevent sustained transmission.

State health officials have urged residents to verify their vaccination status and to contact a health care provider if they suspect exposure. Symptoms typically appear 10 to 14 days after exposure and include high fever, cough, runny nose, red eyes and a distinctive rash that spreads from the face downward.

The department has not yet released additional details about the newly reported cases or their connection to existing clusters. Local health departments in the affected counties are conducting contact tracing and monitoring for further spread.

The outbreak's toll in Grant County, which now accounts for more than a quarter of the state's cases, has strained local health resources and prompted renewed attention to vaccination outreach in rural communities where access to care can be limited.

This story will be updated as more information becomes available.