Why would a doctor prescribe prednisone when a patient has an infection?

Balancing act: It's a balancing act since many of the problems with infections is that an exuberant inflammatory response can occur, which can cause more damage than the infectious agent itself (for example, brain swelling that occurs with encephalitis). Prednisone dampens the immune response while the anti-infective agent can do its job by controlling the microbe causing infection.

Not usually used: Most infections are not treated with steroids since many times steroids can make infections worse. On rare occassions, steroid nose sprays are used in the treatment of sinus infections. In rare brain or CNS inections, steroids can be of benefit. I think most physicians would agree that steroids are not typically used for infection.