I've been using this pen name for about twenty years or so, since I first encountered it a Biology lesson.
Bilharzia is another name for river blindness, a disease caused by a parasite that is fairly common in the tropics. The parasite is the blood fluke, of the genus Schistosoma, and also causes Schistosomiasis, but that's a bit harder to turn into a pen name. So, noting the obvious way to turn it into a name, and being amused (in typical teenage fashion) that I'd be masquerading as a tropical disease, I adopted it.
I've also gone by Billy Rubin once or twice, because bilirubin is the primary component of bile (a name best reserved for vituperative writing), and when I can't get any other alias accepted I fall back on Kwashiorkor (a severe protein deficiency in children).
Looking back, that biology lesson has a lot to answer for....