parasitism-

Parasitism is a type of non mutual relationship between organisms of different species where one organism, the parasite, benefits at the expense the host. Traditionally parasite referred to organisms with lifestages that needed more than one host. These are now called microparasites. The word parasite now also refers to micrparasites, which are typically smaller, such as viruses and bacteria, and can be directly transmitted between hosts of the same species. Some examples of parasites are the plants such as mistletoe and cuscuta and organisms such as leeches.
external image Leech.jpg