So, I am currently investigating parasites, hyperparasites, parasitic castration, and other things that make readers go "ick". (Consider this a research question.)
I'm going to ignore bacterial and protozoal parasites for now, because my Toxoplasma gondii master tells me they're of no interest. And we all know about the mundane visible-with-the-naked-eye ones like tapeworms and pinworms, and the horrifyingly nasty Guinea worm that causes Dracunculiasis. But these are relatively straightforward parasites. Booooring.
However, parasites in general are fascinating and some of their variants are just totally bugfuck. Literally. Take Ophiocordyceps unilateralis, for example. It's a fungal parasite. Its spores hatch and brain-control its host—forest floor dwelling ants—makes them climb high up in the vegetation canopy, then digests its host's body and forms fruiting bodies to scatter new spores over the forest floor for more unfortunate ants to stumble across.
Or take the strange case of the hyperparasitoid wasps—wasps that lay their eggs in wasps that parasitize caterpillars: small cabbage white butterfly caterpillars are used as a host species by the parasitoid wasps—Cotesia rubecula and Cotesia glomerata—which in turn make a handly meal for the even smaller hyperparasitoid wasp Lysibia nana. (Thereby confirming something I've known since I was two years old: wasps are assholes.)
And then things get weird, because these are instances of parasitism where the parasitic life cycle is proceeding according to the script. When parasites go wrong, things get weird-ugly, really fast. Tapeworms sometimes run amok and end up encysting in the brains of their hosts. And then there was the recent case where an HIV-positive man contracted and died of cancer from a non-human source, namely an invasive tapeworm's own neoplasm (which became highly invasive in the immunocompromised host).
Anyway, I thought I should share the joy with you because I'm currently inventing semi-plausible parasitic and meta-parasitic lifecycles with humans as the basic host, for a future Laundry Files story (because Equoids are really just a little passé). And I thought you might like to share some of your favourite parasites and hyperparasites with me! (Nothing well-known or mundane, of course. Assume I'm already familiar with the common stuff.)
Anyone want to start?