Neurodegenerative diseases such as Alzheimer's disease or Parkinson's disease are triggered by harmful aggregation of certain proteins. In their physiological state, the proteins are usually present as monomers. In this state, these proteins fulfill important biological functions.
If the monomers aggregate, e.g. spontaneously with increasing age, harmful oligomers are formed. These have toxic properties and damage neurons and other cells in the brain. Growth of the oligomers leads to insoluble fibrils, which are, however, largely harmless.
In contrast, oligomers have the property that they are soluble, i.e., they can spread in the brain. They can act as a template for other monomers, leading to increased formation of more oligomers. They are said to proliferate in a prion-like manner. Thus, there is a progressive expansion of deleterious effects in the brain, neuronal death, and impairments in, for example, learning and memory.