What are parasites?

Parasites are defined as living organisms that live on or in a host organism and derive their nutrients at the host's expense. They can either be internal, residing within the host's body (such as worms), or external, living on the surface of the host (like ticks and fleas). This characteristic of survival and dependence on a host distinguishes parasites from other types of microorganisms, such as pathogens or infections caused by bacteria or viruses.

This understanding helps to highlight the complexity of the relationship between parasites and their hosts, where the health of the host can be significantly impacted by the presence of the parasite. In contrast, the other definitions do not adequately capture the essence of what makes a parasite. For example, while pathogens may refer to a variety of disease-causing organisms, not all of them rely on a host for survival. Similarly, stating that they are single-celled organisms or that they multiply independently does not align with the typical nature of parasitism. Ultimately, recognizing parasites as living organisms that exploit hosts is crucial in the study of animal health and disease management.