Microorganism or Microbes
Living forms that are too small to be perceptible by the human eye. The scientific study of these is called microbiology: bacterias, fungus, virus, protozoans??
Parasite
An organism whose existence is at the expense of another living organism called the host. The scientific study calls these parasitology.
Ectoparasites: these are located in the coat/skin or hair of the horse, for example fleas, ticks, lice, nits, flies and mites (scabies). They have no importance in cutaneous lesions, but they do produce itching and other effects.
We must also take them into account as vectors or carriers of other diseases:
Ex: The tick is an intermediate host in the cycle of babesiasis. (Blue-tongue)
Endoparasites: Internal parasites that are the principle cause of colic in horses and they are often the causal factor that contributes in the presentation of various problems of respiratory types, digestive and zootechnical function loss.
The five internal parasites that are more commonly found and represent major risk issues of health in horses are the large and small strongyles, large roundworms (ascaris), pinworms (oxyuris equi), and bots (gastrophilus larvae).