A parasite is a plant or animal that feeds on another animal or plant. For example, some plants in the forest rely on animals for food, and they use the animals as a method of transportation to carry food back to their roots. It is this type of growth that gives a plant its nutrition. In a way, a parasite is like a plant that is dead and rotting inside itself. These plants and animals must have oxygen to survive, so parasites must come from somewhere.
The term "parasite" was originally used by Darwin in reference to animals that came from outside of the body, usually through a wound. A well known example of this is the ticks found in our skin, which are very small and unnoticeable. In reality, we are all parasitically linked with each other, to some degree.
The whole idea of parasitism has changed over the years. It is common today to refer to plants as parasites instead of how it used to be. Some plants can be a part of the host plant, and even benefit from them. Many animals that use plants for shelter, to build their nests or other reasons are parasites, and they are of course a form of life. There are parasites of plants that produce various types of seeds or wood, and these make up the host plants' living.
Parasites and plants can be formed from two different forms of life. There are bacteria that can form from an animal or plant, or from bacteria that are already present. Parasites can also form from bacteria that are not yet established in the host's life, but has just started to form. This is similar to how a worm is not yet in the body, and it will grow into a larger form of life after it has begun to alter the structure of the body.
There are those animals and plants that feed off of each other. These are called copepods and bacteria, and many of these are parasitic creatures. A copepod is when an animal and a plant get together and form a strong bond.
There are some forms of life that can go from being a parasite to being a host. The Eukaryote Plant is one example. This is when an animal or plant that is not yet capable of doing without another can become an intermediary between the two.
Another movie about parasites that makes me think about the concept is Outbreak. The movie is based around a virus that is developed by a group of people that becomes self-propagating, causing mass outbreaks. The movie discusses how such a group can cause a virus to get airborne and then travel to other people, infecting them, until the disease becomes widespread.
It is interesting to note that in this movie, a bunch of people become a virus. It can also be said that the viruses are the parasites that are doing the feeding off of the people. If I was trying to use the movie as a way to discuss the topic of parasitism, the movie Outbreak could be used as a comparison. This may not be something you would want to discuss at your next cocktail party, but you would be able to see the basis of the movie with the concept of parasitism.