Coming into contact with a Forgotten Beast is an almost sure way to contract the plague and get contaminants all over one's equipment. When a soldier dies of the plague, other soldiers will look to upgrade their equipment. If they're not stopped, the survivors will pick up the equipment which is inevitably coated in contaminants, and instantly contract the plague themselves when they do. Then they'll die of plague, and the process will repeat.
There are two different ways I know of to prevent this: 1. Forbid all dropped items upon death, and never let a dwarf step near that tile again (if it's contaminated) - or 2. Just don't let dwarves or animals come into contact with forgotten beasts. The first method works, but is impractical since the only way to stop dwarves from deciding to step on contaminated equipment is to encase the items in obsidian or ice, or remove an area about 5 tiles in every direction from the citizens' burrow.
So I use the second method: My caverns are completely sealed (including from flying beasts) except for one destroyable entrance, protected by lever-operated upright spikes. Normal, highly-safety-oriented players (not me) just seal the caverns completely and call it done, I think, letting forgotten beasts accumulate and roam around. Forgotten beasts can't destroy walls or floors--so this works great for them.
I use a slightly-less-safe entrances-with-traps method, in which most forgotten beasts die of being impaled by mechanical spikes while they try to tear down floodgates. This way, I can access the caverns and continue to harvest wood and water from them, and build plumbing structures that go through them, without my builders having to worry about 20 forgotten beasts roaming around.
In other words, I know I'm shooting myself in the foot by letting the caverns be part of my gameplay. But why play without enjoying the caverns? If I'm not living on the edge, I feel I'm taking up too much space.