S: WHO – http://www.who.int/trypanosomiasis_african/parasite/en/ (last access: 10 May 2016); CDC – http://www.cdc.gov/parasites/ (last access: 10 May 2016).
N: 1. parasite (n): 1530s, “a hanger-on, a toady, person who lives on others,” from Middle French parasite (16c.) or directly from Latin parasitus “toady, sponger,” and directly from Greek parasitos “one who lives at another’s expense, person who eats at the table of another,” from noun use of an adjective meaning “feeding beside,” from para- “beside” + sitos “food,” of unknown origin. Scientific meaning “animal or plant that lives on others” is first recorded 1640s (implied in parasitical).
parasitic (adj): 1620s, from Latin parasiticus, from Greek parasitikos “of or pertaining to a parasite; the trade of a parasite,” from parasitos (see parasite). Biological sense is from 1731. Related: Parasitical, 1570s in reference to toadies; from 1640s in the biological sense.
2. A parasite is an organism, or living thing, that lives on or inside another organism. It depends on the other organism for food and other things that it needs to live. The parasite’s victim is called its host. The host is usually much larger than the parasite.
3. Different kinds of parasites have different effects on their hosts. Some parasites cause disease. Others cause pain. Others are almost never noticed by their hosts. However, the basic relationship is always the same—good for the parasite, harmful for the host.
S: 1. OED – http://goo.gl/kx7dW1; http://goo.gl/SjH72U (last access: 10 May 2016). 2 & 3. Kidsbrit – http://kids.britannica.com/elementary/article-390254/parasite (last access: 10 May 2016).