> It sounds like the basic (?) security practices recommended by professionals - keep systems up-to-date, pay attention to whether an email is suspicious - would have covered your network.
This is secondhand information (so take it for what it's worth, there could be pieces I'm missing), but I talked with a startup that was focusing on this problem, and the issue was not quite the computers and servers that IT were using (although sometimes it was), it was that many medical devices (like CT scanners, pumps, etc) come shipped with old outdated versions of operating systems and libraries.
No big deal right? Just make sure those are up to date too? Well, many times the support contract for these medical devices are so strict that you can invalidate the warranty by installing third party software like an antivirus, or even doing something like Windows update.
Even worse, many hospitals don't even know what devices they have -- it's easy for IT to know about laptops and computers, but when every single medical device more complicated than a stethoscope has a chip in it and may respond to calls on certain ports, it's a tougher picture to know.
The startup was https://www.virtalabs.com/ by the way, they really are doing some cool things to help with this.