The Black Magician(1986)
(The second book in the Cunningham series)
A novel by G C Edmondson and C M Kotlan
From the Back Cover: The Blight of the Living Dead. Worm Brains they were called - thousands of people who'd bough an untested treatment guaranteed to produce greater intelligence. And it worked! But after a few weeks of increased IQ, the customers began to die as a parasite slowly grew in their brains. Treating the disease was easy - a few drops of common insecticide would halt the creatures growth. No problem at all, really. Except that the government was hiding the treatment and thousands of victims were dying of malign neglect. And with each death, another thing was born...
Used availability for G C Edmondson's The Black Magician
October 1986 : USA Mass Market Paperback