The brain can be damaged in a variety of ways, and depending on the areas damaged and the severity of the damage

Genetics 

 A dysfunctional hereditary gene could have been passed on to the offspring which prevented the full development of a healthy brain 

Blow 

A sufficient blow to the head can supercede the skulls defences (particularly at the temple) and can therefore allow structural damage to occur.

Lack of Blood 

Lack of blood to the brain can cause severe problems for the cells associated with the brain. A human can survive for four minutes without oxygen before the brain damage becomes so severe there is no realistic chance of survival. A stroke is an event where there is a blood shortage to the brain, which is caused by a blood clot. 

Tumours 

Cancer has been a major non-infectious disease more recognised over the last decade, and more cases of brain tumours are detected nowadays due to more sophisticated techniques. The continued growth of these cancerous cells puts pressure on the brain, which can cause a blood clot or directly cause brain damage due to the pressure of the tumour pressing against it.