The old saying goes: "Starve a fever; feed a cold."
Dutch researchers have actually confirmed that nutritional status does have an effect on the regulation of the immune response. Apparently, different types of immune cells increase in numbers according to whether one is or is not fed a (presumably nutritious) meal.
When, after overnight fasting, volunteers were fed a liquid meal, their levels of gamma interferon increased 6-fold. In other volunteers fed nothing but water, interleukin-4 nearly quadrupled. While gamma interferon is a hallmark of the type of immune response directed against viral infections (e.g. the common cold), interleukin-4 is characteristic of the immune response directed against most bacterial infections (responsible for most fevers).
I guess those old wives knew what they were talking about.