Bioweapons and the like are under extraordinarily paranoid controls. Even a panicking scientist -- or group thereof -- practically has to try really hard to get it out into the wild.
However, secondary diseases would almost certainly be a reality. As society breaks down, we're going to lose access to clean water, and see a spike in waterborne illnesses. Medical infrastructure will fail, so things like the "regular" flu will become more serious issues and will more readily spread with the loss of access to annual vaccinations. Diseases that are fairly common but easily treatable will still be at least as common, but become much more deadly -- pneumonia, for instance, can usually be treated well in a hospital and has a high survivability as a result, but without access to hospital treatment the odds of someone who catches it surviving dwindle rapidly (further compounded by lack of clean water, food, etc.).
Measles, mumps, and rubella will return, albeit not for a few years (gotta work out the "herd immunity" of those already immunized), and will again be serious illnesses.
On the other hand, with fewer people (presumably spread out across vast distances), widespread pandemics would become very difficult -- but individual enclaves could easily be wiped out entirely (or at the least suffer local epidemics) by diseases you or I today consider minor inconveniences at best.
The end result is that if your disease wipes out 90%, you can pretty much bet that a good chunk of those who somehow survive yours will nonetheless succumb to one or more of the many other diseases that suddenly become big deals again.