What type of organisms cause diseases such as head lice and nail fungus?

External parasites are organisms that live on or in a host organism, feeding off the host's resources. Head lice, for example, are external parasites that infest the hair and scalp, leading to discomfort and irritation. They thrive by consuming blood from the host. Nail fungus, while it is caused by fungi, can also be considered a type of organism that lives on the surface and penetrates the nail, indirectly relating it to the concept of external surroundings of the host.

In contrast, internal parasites live within the host and typically affect the host's bodily systems from the inside. Bacteria and viruses, on the other hand, are microscopic organisms that can cause disease but do not fit the definition of external parasites; they often require different treatment and preventive measures. Understanding these distinctions is essential for identifying causes and implementing proper treatment for infestations and infections.