What type of diseases do protozoa primarily cause?

Protozoa are single-celled eukaryotic organisms that can cause a variety of diseases primarily categorized as parasitic diseases. These organisms are capable of living and reproducing within their hosts, often leading to significant health issues. Common parasitic diseases caused by protozoa include malaria, amoebic dysentery, and sleeping sickness, among others.

The nature of these diseases typically involves the protozoa invading host tissues or utilizing host resources for their own survival, which can result in inflammation, tissue damage, and disruption of normal physiological functions. The classification of these diseases as parasitic highlights the relationship between the protozoa and their hosts, which is a critical aspect in the study of protozoan pathogens.

In contrast, the other options do not accurately reflect the nature of diseases caused by protozoa. Bacterial infections are caused by bacteria, viral infections are caused by viruses, and fungal infections are caused by fungi. Each of these categories involves distinct pathogens that operate under different mechanisms of disease. Thus, C is the correct choice as it accurately describes the primary type of diseases caused by protozoa.