What type of pathogen is often visible and can infect other organisms?

The correct answer indicates that both parasites and bacteria fall into the category of pathogens that can often be visible and have the ability to infect other organisms.

Parasites are organisms that live on or in a host and benefit at the host's expense. Many parasites, such as certain worms and protozoa, can often be seen with the naked eye or with minimal magnification. They can infect a wide range of hosts, including humans and animals, leading to various health issues.

Bacteria, though single-celled microorganisms, can also be large enough to be observed using a microscope and play a role as pathogens. Certain bacteria can cause infections in organisms, leading to diseases. While some bacteria are harmless or even beneficial, pathogenic bacteria can have significant effects on health, agriculture, and the environment.

By recognizing both parasites and bacteria as visible pathogens that can infect hosts, this answer encompasses a broader understanding of the types of infectious agents in both animal and human health contexts.