From a medical standpoint, parasites describe a number of multi-cellular organisms that are able to survive on or within the human body, but have side effects that are harmful to human health. Technically speaking, a parasite has two characteristics:
- Part of its life cycle must be spent within a host body; and
- It requires sustenance from the host in order to survive during this period.
However, the term 'parasite' is usually used to refer to any organism that meets either of these characteristics.
Parasites do not include single-celled organisms, such as bacteria, amoeba or viruses that can cause disease. It also excludes multi-cellular creatures that normunting.