Actually, the parasite system is far more complex than it appears. Parasites are composed of complete eukaryotic cells in opposition to the simpler prokaryotic cell composing bacteria. This type of cells also composed animals and plants. Eukaryotic cells have a full set of organelles, including a nucleus to protect the genetic materials and mitochondria to produce energy. They are considered as quite complex and evolved cells.
Even though protozoan parasites are unicellular, meaning that they are composed of only one cell, they are quite evolved organisms, and they are even able to manipulate the immune system of their host. On the other hand, helminths are usually multicellular organisms, meaning that they are composed of many cells. Even though these cells are less specialized than the ones found in higher animals, they are considered as small primitive animals. It is noteworthy that helminths are also able to manipulate their host body responses, such as the functioning of the immune system.
Actually, parasites have all they need in their cell(s) to survive as independent living organisms. In fact, they are able to do perfectly all the functions associated to living organisms, such as reproduction, nutrition and relationship between different individuals. However, for normal people, the reproduction and the nutrition parts are easier to understand than the relationship part. The following paragraphs will explain more in details these three functions.
Depending on the species, parasites reproduce themselves by some asexual means or by sexual means. Asexual reproduction is the most frequently observed form occurring in single-celled organisms, such as protozoan parasites. It involves the division of one cell in order to generate an offspring. However, in some cases, genetic material exchanges typical of sexual reproduction have been shown to occur in some protozoan parasites. For instance, this phenomenon of sexual reproduction has been proven to occur between different Leishmania individuals within their sandfly insect vector. On the other hand, multicellular organisms, such as the majority of the helminths, harbour sexual reproduction involving an exchange of genetic materials between two individuals. It is interesting to note that some helminths, such as tapeworms and the majority of flukes, are hermaphrodites, meaning that they have simultaneously both sexes, while other helminths, such as roundworms and the blood-fluke Schistosoma, have distinct female and male forms. Furthermore, helminths also have a complex life cycle involving eggs and many larval stages.
The nutrition of pathogenic parasites is usually closely related to their hosts. Actually, they are able to ingest specific nutrients taken from their hosts.
Relationships between parasites are far more complex than what people usually think. In fact, they are able to secrete different molecules in order to attract (when their environment is favourable for them) or repulse (when their environment is not adequate or toxic) other parasites from the same species. When infecting a cell, they are even able to take control of it in order to make the cell releases molecules attracting other parasites or other cells to infect in case the former one burst and they need to infect another one.
Finally, even though they look simple, parasites are far more complex organisms than people usually think.
Sources: Centers for Disease Control and Prevention; Akopyants, N. S. et al. (2009). Demonstration of genetic exchange during cyclical development of Leishmania in the sand fly vector. Science, 324(5924), 265-268.