Apart from bacterial (don’t forget Tuberculosis) & viral (don’t forget HIV) causes, infection of the nervous system can also be caused by parasites (Toxoplamosis), fungi (Cryptococcus) and spirochaetes (Syphilis & Borreliosis or Lyme disease). Toxins produced by some bacteria can also cause disease – Tetanus, Botulism and Diphtheria.

1. Meningitis

This is inflammation of the coverings of the brain, it may be lethal or cause significant residual neurological disability. The most aggressive type is caused by bacteria and nowadays is due either to "the meningococcus" or "the pneumococcus". Milder forms are caused by a variety of different viruses whereas more chronic & indolent infection makes one think of tuberculosis, fungi and cancer as potential causes.

Symptoms tend to present acutely over a day or so or more catastrophically over just a few hours, and include headache, neck stiffness, fear of light, fever, nausea, vomiting, confusion, irritability and body rash.

Management is a medical emergency. Antibiotics are often given at low threshold by your General Practitioner at initial contact before urgent transfer to hospital. Brain Imaging, usually with CT Scanning, along with blood investigations, precede cerebrospinal fluid (CSF) examination at lumbar puncture. CSF analysis provides accurate microbiological diagnosis to guide treatment with intravenous antibiotics.

2. Encephalitis

This is inflammation of the brain substance itself and like meningitis is life-threatening. Unlike meningitis, this is usually caused by a virus with the Herpes Simplex virus being the commonest agent in the United Kingdom. Meningitic features (see above) are the usual presenting symptoms with speech disturbance, seizures and limb weakness affecting one-or-other side of the body developing over some days, all superimposed upon a background of drowsiness and progressive loss of consciousness.

Investigation is with CT Scanning and CSF examination as above but greater use of MR Imaging of brain and Electroencephalography (EEG) in this setting can be particularly helpful. Early use of antiviral drugs is imperative to avoid significant residual disability.

3. Brain Abscess

This is a collection of pus in the brain and is also potentially fatal. Pus may gain access to the brain from a local structure such as the ear or a paranasal sinus. Infection may enter the brain via a penetrating head injury or spread in the blood often from disease of the heart valves (infective endocarditis) or from pus in the chest.

In addition to the investigations above, tests may include x-rays of skull & chest and culturing of the blood. CSF examination however is usually dangerous and not performed.

Treatment is with aggressive and often prolonged courses of antibiotics frequently in combination. In difficult cases and with isolated large abscess cavities, pus can be aspirated at neurosurgery. Attention to the underlying source of pus is imperative and may require lavage or drainage of paranasal sinuses or chest & even open heart surgery with valve replacement in some cases.