As influenza season approaches, some Americans, and especially parents, are worried that, if they or their children should become ill, it may not be easy to know which disease they have — the flu or Covid-19. They are correct. Most symptoms of the two diseases are so similar that, short of a test — or two or three tests — it won’t be possible to know for sure. But there are some clues. (And it is possible to have both infections at the same time; some patients in China this year were found to have both.) But first: get a flu shot. It is not yet clear whether the United States will have much of a flu season this year. Flu activity in the Southern Hemisphere, which is often predictive of activity in the United States, was 99 percent below normal during its winter. Epidemiologists believe that is because Australians, New Zealanders, South Africans, Chileans and other residents of the southern half of the globe were wearing masks, staying several feet apart and washing their hands to prev...