"Walking pneumonia" sounds like it could be a character in a sci-fi horror flick. Although this form of infectious pneumonia can make you miserable, it's actually the least scary kind of pneumonia. That's because it's a mild pneumonia and does not generally require hospitalization. In fact, you could have walking pneumonia and not even know it. Here is information about what causes this illness, how it spreads, and what you can do to avoid it.