Whereas the effect of caffeine on metabolism via CYP enzymes is disputed—some sources state that caffeine may lead to an increase of clozapine concentrations [33,242,243] with others claiming caffeine to be merely a substrate of CYP1A2 [108]—smoking habits (which often change during infection, especially respiratory infection) have a relevant effect on the CYP enzyme system, as delineated in Section 2.2. The gene discussed is PPIG; the disease is infection.