Food workers who showed up while sick or contagious were linked to about 40 percent of restaurant food poisoning outbreaks with a known cause between 2017 and 2019, federal health officials reported.

Norovirus and salmonella, germs that can cause severe illness, were the most common cause of 800 outbreaks, which encompassed 875 restaurants and were reported by 25 state and local health departments.

Investigators with the Centers for Disease Control and Prevention called for better enforcement of “comprehensive food safety policies,” which emphasize basic measures like hand washing and keeping sick workers off the job.

Although 85 percent of restaurants said they had policies restricting staff from working while sick, only about 16 percent of the policies were detailed enough to require workers to notify managers and to stay home if they had any of the five key symptoms—including vomiting, diarrhea and sore throat with fever.

About 44 percent of managers told the CDC their restaurants provided paid sick leave to workers. That is a problem, according to Mitzi Baum, the chief executive of STOP Foodborne Illness, a nonprofit advocacy group.

She said it means workers are forced to choose between earning money or showing up sick—or there is social pressure not to leave fellow employees short-staffed.

“If there’s a positive food safety culture, you’re not penalized for illness,” Ms. Baum said.

It can be hard for consumers to know when sick workers might be on the job, she said, but there are some signs to look for: “Is your server sniffing? Are they sneezing? How are they handling the utensils?”

About 48 million people a year in the U.S. are sickened by foodborne illness, including 128,000 who are hospitalized and 3,000 who die, according to the CDC.

This report contains information from The Associated Press.