Why are viruses considered a type of pathogen?

Viruses are classified as pathogens because they have the ability to infect living cells, including those in humans, animals, and plants, leading to various diseases. When it comes to water, certain viruses can contaminate water supplies and pose a significant risk to public health. This is particularly concerning in drinking water treatment and distribution, as some viral infections can occur even at low concentrations. As pathogens, they can be resilient in the environment, and traditional water treatment methods may not always effectively eliminate them, making it essential for water facilities to implement additional barriers for virus removal and inactivation.

The other options do not accurately describe the nature of viruses in relation to public health and water safety. While some microorganisms, including certain bacteria, can play a beneficial role in ecosystems or water quality, viruses do not fit this description. They are not typically associated with beneficial effects for humans and are not easily removed by standard filtration processes, which can make them a particular concern for drinking water systems.