There was only one mention of an infectious disease in the novel and it involved Colonel Jean Sandherr, the head of counter-espionage for the French military responsible for the false case against Alfred Dreyfus.
Harris describes Sandherr as suffering from general paresis caused by syphilis. This complication occurs in late stage syphilis in which the invading spirochete, Treponema palladium, has caused a chronic meningitis that leads to degenerative changes in the brain.
Every so often, I get consulted on a case of an elderly individual being worked up for dementia because, as part of that workup, a syphilis test is ordered and returns positive. While I've never seen a case of general paresis, these individuals have "late latent" syphilis and undergo treatment (which never really changes the course of their dementia).
Common in earlier eras, this stage of syphilis is quite rare today. However, the earlier stages of syphilis still abound and, in recent years, the bacterium has become tech-savvy as it has now begun to use social-networking sex apps to find new victims. It seems to have found success as cases doubled between 2005 and 2013, primarily in men who have sex with men.
At least there's no worry about syphilis being bewildered in the modern digital world because included in the internet of everything are the every resilient STDs.