Mental illnesses of short duration are generally considered to be "acute".
In medicine, portraying a sickness as acute signifies that it is of brief term and, as a result of that, of recent beginning. The quantitation of how much time comprises "short" and "recent" shifts by infection and by setting of the disease, however the main meaning of "acute" is in every case subjectively in opposition with "chronic", which indicates an old and long sickness.