Final answer:
Subway air and water distribution systems can harbor high levels of pathogens, especially bacteria, which are a major concern due to their abundance and survival capabilities in these environments.
Step-by-step explanation:
The question is concerned with identifying which type of pathogen is commonly found in high levels within subway air and water distribution systems. These systems can indeed harbor dangerous levels of certain pathogens due to the high volume of people, moisture, and lack of sunlight, which can create an environment conducive to microbial growth. While all the options listed, such as bacteria, viruses, fungi, and parasites, can be found in such environments, research has shown that bacteria are often the most concerning due to their abundance and ability to survive on various surfaces and materials found in subway systems.