Solano County Department of Health on Thursday confirmed that a patient had been diagnosed with both infections.
Health chiefs did not reveal any personal details about the patient other than that she is a woman over the age of 20, works in the healthcare sector, has no underlying health conditions and has since recovered.
Bela Matyas, the Solano County health officer, said: “This is a very clear indication of the potential for this to occur.
"We now have flu in our community at the same time we have COVID ...contracting either disease may weaken your immune system and make you more susceptible to the other disease.”
It was not immediately clear where or how the woman became ill with both Covid and the flu.
Because coronavirus is a new infection, it is not yet known how contracting it and the flu at the same affects the body. Doctors are urging people who are eligible for a flu jab to get one.
Scientists and infectious disease experts have for months been concerned about the prospect of the Covid-19 pandemic converging with flu season as winter approaches.
"It certainly can’t be good to be infected with both and it may well be a greater challenge to the person who has both infections, and that could make the outcomes worse,” said John Swartzberg, an infectious disease expert at Berkeley School of Public Health.
“So we should do everything we can to prevent infections.”
The dual infection came in a week when record levels of new coronavirus cases were reported in the US.
Some 85,000 cases were logged on 23 October - the highest daily total since the pandemic struck in March.
On Friday that record was broken when almost 100,000 new infections were reported, pushing the overall number of cases over 9 million.
Some 230,000 deaths have been recorded across the country.