I was involved in this discussion a while back on another message board. Some people were very offended and angered at the use of the term parasite to describe the relationship of a mammalian fetus to its mother. Parasite is a derogatory term when used in a metaphorical sense, as in calling somebody a societal parasite.
But in the biological sense, the term is neutral, not judgmental, and merely describes the relationship between two creatures. When one benefits and the other benefits as well, biologists call the relationship mutualism. If one benefits and the other is unaffected, it is commensalism. And if one benefits at the cost of the other, it's parasitism. No judgment, just an objective description. These definitions come from assorted medical dictionaries:
[1] "A plant or animal that lives upon or within another living organism at whose expense it obtains some advantage." - Dorland's Medical Dictionary for Health Consumers
[2] "An organism that grows, feeds, and is sheltered on or in a different organism while contributing nothing to the survival of its host." - The American Heritage Medical Dictionary
[3] "An organism that lives in or with another organism, called the host, in parasitism, a type of association characterized by the parasite obtaining benefits from the host, such as food, and the host being injured as a result." - Gale Encyclopedia of Medicine
[4] "An organism living in or on and obtaining nourishment from another organism." - Mosby's Medical Dictionary
The discussion went on for weeks, and was very heated. People accused others of using the word to describe a fetus just to dehumanize it and justify abortion. It was pointed out that even when the mother was excited to be pregnant and anxious to bear a child, the relationship between these two organisms still qualified as parasitic, and that there was no need to have an emotional reaction, but that didn't matter.
It was noted that the science of parasitology has its own understanding of what constitutes a parasite. The parasite needs to be of another species than the host, and had to cause disease, not just drain resources. The angry posters insisted that only this definition and the derogatory metaphorical one were valid, and continued to object strenuously and angrily to a fetus being classified as a parasite.