Now researchers have discovered yet another unpleasant surprise. In many patients hospitalized with the coronavirus, the immune system is threatened by a depletion of certain essential cells, suggesting eerie parallels with H.I.V.

The findings suggest that a popular treatment to tamp down the immune system in severely ill patients may help a few, but could harm many others. The research offers clues about why very few children get sick when they are infected, and hints that a cocktail of drugs may be needed to bring the coronavirus under control, as is the case with H.I.V.

Growing research points to “very complex immunological signatures of the virus,” said Dr. John Wherry, an immunologist at the University of Pennsylvania whose lab is taking a detailed look at the immune systems of Covid-19 patients.