Which of the following is a type of waterborne pathogen?

Legionnaires' disease is classified as a waterborne pathogen because it is caused by the bacteria Legionella, which is typically found in freshwater environments like rivers and lakes, as well as in man-made water systems such as hot tubs, cooling towers, and plumbing systems. When aerosolized water containing these bacteria is inhaled, it can lead to severe respiratory illness, specifically pneumonia-like symptoms, which characterize Legionnaires' disease.

In contrast, the other conditions listed, such as the common cold, flu, and pneumonia, are primarily viral and respiratory infections that are not contracted through water sources. The common cold and flu are caused by viruses that spread through respiratory droplets when an infected person coughs or sneezes, whereas pneumonia is an infection that can be caused by various pathogens including bacteria and viruses, but not specifically classified as waterborne in the same manner as Legionnaires' disease. Therefore, Legionnaires' disease stands out as the only condition in this group primarily associated with contamination through water.