I agree with the others, but I think we should note that these sorts of illness are not divided neatly into two categories, cold and flu. Besides the many different viruses grouped under "the common cold", there is a whole spectrum of bacterial infections whose symptoms look similar to, and interpolate between, the cold and flu. So there are other things someone could get that are like less severe version of the flu, but are worse than a common cold, and it's hard to distinguish without a formal test.

I believe pneumonia can be caused by a number of bacteria. These used to be very deadly before the advance of antibiotics, much worse than the flu or cold.