when someone gets a brain infection/disease/abcess, is it through air or germs or bacteria, how do they get it?

Through virus or infection and there are 3 ways this can happen:

Infection is spread from another area of infection in the body.

Infection is spread thru the bloodstream from the lung or chest area.

Viral or bacterial germs enter directly into the brain through an open wound in the head.

RISK FACTORS:

Congenital heart disease (present at birth)

Meningitis

Chronic middle ear and sinus infections

Dental or jaw infections

Head injury of skull fracture

Infections of the face or scalp

In some people the thick membrane that protects the brain is not as effective as it should be and infections in the blood can permeate through causing meningitis. In babies the bug that causes pneumonia can cause meningitis as their immune systems have not built up.

An infection in the cerebro spinal fluid (CSF surrounds and supports the brain and spinal column) can be caused by spinal injury or surgery.

Infections are caused by both bacteria and viruses.