Which disease cannot be contracted by drinking contaminated water?

Malaria is transmitted via the bite of an infected mosquito, specifically the Anopheles species, and is not spread through contaminated water. This distinguishes it from the other diseases listed, which are directly linked to water contamination.

Typhoid fever is caused by the bacterium Salmonella Typhi, which can be spread through drinking water contaminated with fecal material. Giardia, caused by the protozoan parasite Giardia lamblia, is transmitted when contaminated water is consumed, especially in areas with poor sanitation. Legionnaires' disease, associated with the Legionella bacterium, can be linked to water systems like hot tubs, cooling towers, or plumbing systems, where the bacteria can multiply in stagnant water, leading to infection when the contaminated water is aerosolized and inhaled.

Understanding these modes of transmission highlights that while typhoid, Giardia, and Legionnaires' disease can all be contracted through contaminated water, malaria's vector is entirely different, thus making it the correct answer in this context.