Mayor Sumbul Siddiqui, left, and Meryl Brott from the Department of Public Works visited the Amigos and Cambridgeport schools Tuesday to talk about recycling. (Photo: Mayor Sumbul Siddiqui via Twitter)

With Covid-19 cases continuing to rise, the Cambridge Public Health Department on Monday called for indoor masking more forcefully than before, and school superintendent Victoria L. Greer said the district is “encouraging our entire school community to mask, particularly when we are indoors.”

The messages came as infections and hospitalizations brought Middlesex County into the high-transmission category under Centers for Disease Control and Prevention definitions; on Thursday, the City of Somerville noted the change to residents with a texted and emailed message that “strongly urges that you wear a high-quality mask when indoors in public.”

Cambridge’s health department “strongly recommends that residents wear a mask when indoors outside the home,” according to a press release. The masking advice follows CDC guidance; the department also repeated the rest of the agency’s advice for people in high-transmission counties, including getting recommended Covid vaccinations, getting tested “if you have symptoms” and staying home if you’re sick. People at high risk of serious illness from the virus should take more precautions, the department said.

On Monday, the health department reported 304 new cases from Friday, Saturday and Sunday combined; Cambridge moved to daily data reports from Monday to Friday starting Saturday. Health officials in Cambridge and elsewhere in the country have said that reported case numbers are probably understating infections because more people are testing themselves at home and most of those results don’t go to public health departments.

The school district said its new guidance was “due to an increase in the number of Covid cases and Covid-related hospitalizations within our community.”

The health department message repeats and intensifies slightly one from April 22, when the department applauded the fact that many residents continue to mask up, saying: “The Cambridge Public Health Department strongly supports this practice!” It recommended that people “wear a mask when indoors outside your own home, including on public transportation.”