You may be thinking of numerous stories about people with brain tumours having higher IQ which I think are entirely fictional. The brain is a very complex organ with lots of functions (most of them not linked to intelligence at all) and we actually understand very little of how it works. Usually disease lets us work out something about how the brain works because, when that bit is damaged the person can't do something (the example often cited being the emotional and personality changes in Phineas Gage or the effect of tumour on Broca's area or the effect of severing the hemispheres on perception - a treatment that used to be used for epilepsy and it causes some weird effects). Therefore, brain damage of any form usually has bad effects rather than good.
I would also be careful of citing IQ as a measure of intelligence. It isn't, it measures spatial and numeric reasoning (which is part of intelligence but not all of it) and it is most commonly considered among modern psychologists that 'intelligence' is a more complex and multi-factorial factor than a mere numerical scale of problem solving ability can represent*. Have a look at concepts of multiple intelligence theory which includes aspects such as social intelligence, emotional intelligence, linguistic etc. Not sure if this is confirmed by any studies, but I am fairly sure from observation of 'intelligent' people that sometimes having too much of one aspect of intelligence can lead to a deficiency in the others (hence the stereotype of the antisocial geek or the stories you hear about Einstein having real problems relating to other people). Not sure this is necessarily related to a genetic cause, however. More likely it is that someone with a talent in one area can neglect the others in their education and training to focus on developing the one thing they are good at.
*Psychologists are still baffled as to why many people still place so much on IQ as a measure of intelligence, especially companies looking to use them in their hiring process...