Leeches, e.g

Usage examples of parasites.

Next, I put some of the saliva through the centrifuge, just to be sure the parasites and the viruses did indeed separate out at different densities.

That meant the parasites could be transferred to and from an array of other warm-blooded hosts, including rats.

I was too far away to make out the platelet parasites around them, but a sense of diffused light there -- like dust floating -- suggested their presence by the thousands or millions.

Each new mutation led to parasites which could prey on earlier parasites.

When you see the Pope and his parasites, tell them that those to whom I must report did not want the girl alive.

The majority of the born-again and Pax military had deserted outright, seeking out communion to rid themselves of the cruciform parasites and shunning the Pax loyalists.

They had a complete understanding of the altered genetic mechanisms that let the malaria parasites colonize sickled blood cells, where no malaria parasites had ever been able to boldly go before.

Shortly thereafter, Kira is contacted by General Taulin Cyl of the Trill Defense Ministry, who informs her that he has information about the parasites, which are still a clear and present danger not only to Bajor, but the entire Federation.

We now believe the parasites are either slow to spread, or their numbers are small.

Girani answered as best she could, but it was apparent that research on the parasites was still in its earliest stages.

DS9, claiming, as Gard had, that the parasites wanted to destroy Trill.

For all they knew, the parasites meant genocide for the Bajoran people.

DNA codings between the symbionts and the parasites were similar, from the same gene family.

As far I know, no one ever put forth the theory that the parasites were engineered.

I mean is that maybe the reason no one has ever suggested the possibility that the parasites were engineered from symbionts is precisely because no one wanted that to be known.