Which type of microorganism usually does not cause human disease?

Algae are primarily known for their role in aquatic ecosystems and are generally not associated with causing human diseases. While some species can produce toxins (like red tides or harmful algal blooms), the majority of algae are harmless and actually play beneficial roles in the environment, such as oxygen production and serving as a food source for various forms of marine life.

In contrast, bacteria, viruses, and protozoa include many pathogens capable of causing diseases in humans. Bacteria can lead to infections ranging from mild to life-threatening. Viruses are known for causing a wide array of diseases, including the common cold, influenza, and more severe conditions like HIV and COVID-19. Protozoa can also be parasitic, with some species responsible for illnesses such as malaria or giardiasis.