Which condition is characterized by the swelling or infection of the brain?

Encephalitis is characterized by the swelling or infection of the brain, often due to viral or bacterial causes. This condition can lead to inflammation in the brain tissue, which can result in various neurological symptoms such as confusion, seizures, or changes in behavior. The inflammation can also affect cognitive functions and physical coordination.

Meningitis, while related, specifically refers to the inflammation of the protective membranes covering the brain and spinal cord, rather than the brain tissue itself. Fragile X syndrome is a genetic condition that causes intellectual disability and does not involve brain swelling or infection. Spina bifida is a congenital disorder affecting the spinal cord's development, unrelated to brain inflammation or infection. Thus, encephalitis stands out as the condition most directly associated with brain swelling and infection among the given options.