The first thing you should know here is that amnesia is not strictly classified as an illness. It is a temporary disorder, which, if left untreated or unaddressed, may become a long term problem. It is more of a disorder which is triggered by causes that can be roughly divided into two categories. Amnesia is basically a condition wherein the normal functional memory of a human being is disturbed. That is to say that the flow of memory as we understand it gets affected, lost, distorted or damaged. There are two kinds of causes which can lead to amnesia. One kind of cause can be organic, which indicates some sort of damage to the brain. This kind of damage can be caused due to things such as brain damage, injury, excessive trauma to the brain or the incorrect or excessive use of certain kinds of drugs. The other kind of cause is functional, which indicates psychological reasons for the onset of amnesia. In these cases, the amnesia is caused when a person is unable to cope with a great shock, or a loss of some sort. It is also triggered as a defense mechanism to protect the sanity of the person affected, so to speak.
There are also cases of amnesia wherein people will block out whole parts of the past in order to forget it, or to get away from it. Childhood amnesia is the example of one such condition. At times, people will repress events that have occurred in their childhood to such an extent that they eventually erase those memories. These kinds of memories are usually ones which are unpleasant and which they do not want to remember for whatever reason. Amnesia is a treatable problem, but it would require psychological assistance, and, in some cases, hospitalization as well. Sometimes, rehabilitation of the patient also becomes necessary in treating amnesia.
answered by G M