Difficulty of cure
Once a successful fork bomb has been activated in a system, it may not be possible to resume normal operation without rebooting it, as the only solution to a fork bomb is to destroy all instances of it. This is generally not possible since trying to use a program to kill the rogue processes would require another process to be created, which is not possible.
Difficulty of prevention
The way in which a fork bomb functions is to spawn as many processes as possible; thus, to prevent a fork bomb one simply needs to limit the number of processes which may be produced by a single program or user. By only allowing untrusted users to run a relatively small number of processes, the danger of a fork bomb, malicious or unintentional, is reduced. However, this does not prevent the possibility of a group of users collaborating to consume process slots.