People breathe more than 11,000 liters (3,000 gallons) of air every day [source: Nebraska Department of Environmental Quality]. And so, as you would expect, the lungs are a favorite home of bacteria, viruses, fungi and even parasites. And when anything foreign colonizes the lungs, the most common result is fluid. The umbrella term we use to describe fluid in the lungs is pneumonia.
Hippocrates wrote that fluid in the lungs should be called pneumonia if, "the fever be acute, and if there be pains on either side, or in both, and if expiration be if cough be present, and the sputa expectorated be of a blond or livid color" [source: Hippocrates]. But he also distinctly calls it a "disease of the ancients."
Where exactly does pneumonia place in this list of oldest known diseases? Because it's a soft tissue disease, the archaeological record isn't strong. But it's likely that various forms of pneumonia have been around as long as our lungs.