Wabbits are the self replicating computer applications. Wabbits usually don't infect programs and documents of the system, which makes them different from viruses. Computer network capabilities are not used by wabbits to spread, which makes them different from worms. The only purpose of a wabbit is to repeatedly replicate itself on the particular computer. Wabbits are usually programmed to carry on additional (often malicious) effects, apart from the consequences of their quick and repeated self replicating activity. The term "wabbit" must have come from the pronunciation of the word "rabbit", originally created by Elmer Fudd in the "Bugs Bunny" cartoon series. There programs are capable of quick self replicating, very much like the rabbits possess an ability to quickly multiple.
A self replicating computer program is often called a quine. The idea is that once the program is executed, it outputs the code similar to its own thus creating its clone. The clone then initializes itself for the further multiplying and so on and so on. Many programming languages treat empty programs as legal ones, so it is possible to make the program execute itself so that the process becomes unstoppable and the self replicating disaster grows in a blink of an eye, eating your system resources thus slowing down your computer.
It is even scary to imagine what harm can be done by those self replicating applications, if they distribute any dangerous software while replicating themselves. If the process goes too deep, there is nearly any other way apart from eliminating all your operation system so that a wabbit simply has no environment to replicate. Regrettably, even the most up-to-date anti virus and anti malware applications are sometimes helpless when facing a wabbit. That is because this kind of malware is so fast in multiplying itself thus requiring lots of system resources, leaving the combat software far behind.
It would be not honest to say that self replicating applications can carry harm only. They are sometimes used for the useful purposes. For instance, self replicating software is widely used by the scientists to emulate the self replicating processes which can be observed inside our body. Such programs allow the scholars to study the nature of such processes, predict possible problems and thus making them capable of inventing something to fight those problems.
You can throw a parallel between the computer self replicating malware and the viruses, self replicating in our body. They both tend to conquer the whole environment by multiplying themselves in rapid tempo. And yes they both want it to cause harm.
Chances are low for you to get infected by the self replicating software, one of those wabbits. This kind of malware is rare enough as there is actually no mean in repeatedly multiplying a potentially harmless application, and those able to equip a wabbit with something much more malicious obviously have more interesting things to do than attacking a computer of an ordinary user like you. If you are not the Pentagon server system administrator, of course.