The Atom joins Superman and Batman for a nicely balanced story, by Haney, Dillin and Calnan, in World’s Finest 236 (March 1976).
A patient is suffering from a mysterious and downright scary looking virus, attacking his cells.
Superman’s powers are used to get the Atom into the man’s body, while being able to carry a cure. Batman goes off in search of the person spreading the disease.
Everyone gets something to do, with the Atom getting the best visuals. Batman finds the living virus, and discovers that the scary things are actually doing good work, fighting the infected cells.
Superman then has to shrink down and get to the Atom before he can kill the scary but benevolent things.