Encephalitis is sudden inflammation (swelling) in the brain.  It is usually caused by viruses, bacteria, or other pathogens.  As the brain swells, it can get damaged when it gets crushed against the skull.  Encephalitis can cause serious symptoms, like seizures and strokes, and can be fatal.  In 2013, encephalitis killed about 77,000 people in the world.

Signs and symptoms
Usually, adults with encephalitis have a fever that starts suddenly, a headache, confusion, and sometimes seizures.  Younger children or infants may be irritable (easily upset), not want to eat, and have a fever.  Usually, patients are either very tired or confused.

A stiff neck is a sign that the person has either meningitis (inflammation of the meninges, which cover the brain) or meningoencephalitis (swelling of both the meninges and the brain).

Causes

Viruses
Viral encephalitis can happen when a virus infects the brain.  The most common causes are the rabies virus, herpes simplex virus (HSV), the polio virus, and the measles virus.  Viral encephalitis can also be caused by a latent virus - a virus that hides from the body's immune system the brain's nerve cells.  Two examples of viruses that cause latent infection are the varicella-zoster viruses, which can hide in the brain after causing chicken pox, and the herpes simplex virus.

About 100 different viruses can infect the brain.  Other examples include West Nile virus, Chikungunya virus, and Japanese encephalitis virus.

Bacteria
Encephalitis can happen when a person gets a bacterial infection, like bacterial meningitis.  It can also be a complication of an infectious disease that a person already has, like syphilis.  This is called "secondary encephalitis." 

Examples of other bacteria which can cause encephalitis are Staphylococcus aureus, which causes toxic shock syndrome; Bordetella pertussis, which causes pertussis (whooping cough); and types of Borellia bacteria, which cause Lyme disease.

Parasites
Some parasites can infect the brain, especially in people who have weak immune systems.  Examples include Toxoplasma gondii, which causes toxoplasmosis; two parasites from the species Trypanosoma brucei, which cause African trypanosomiasis (African sleeping sickness); and Plasmodium parasites, which cause malaria.

Autoimmune diseases
Autoimmune diseases can cause encephalitis if the body's immune system attacks the brain, the spinal cord, and their nerves.  Two examples are autoimmune encephalitis and acute disseminated encephalitis.

Diagnosis
Encephalitis can be diagnosed in a few different ways:

 An MRI (brain scan) can show brain swelling, and also show whether there are other brain problems causing a patient's symptoms
 An EEG (a picture of the brain's electrical activity) will show signals that are not normal
 A lumbar puncture (spinal tap) will show whether there is an infection in the cerebrospinal fluid (which bathes the brain and spinal cord)
 Blood tests or urine tests may show whether a person has encephalitis, or if they have another illness instead

Treatment
Some treatments for encephalitis depend on the cause:
 Viral encephalitis is treated with anti-viral medications
 Bacterial encephalitis is treated with antibiotic medications

Other treatments are "supportive" - they treat the symptoms of encephalitis.  For example:
 Steroid medications are used to decrease swelling in the brain
 Anticonvulsant medications can stop seizures
 Sedative medications can help with restlessness and irritability
 Acetaminophen can decrease fever and headache
 Physical therapy and speech therapy can help if the person has trouble moving or speaking after their infection is gone

Prevention
Vaccines have made encephalitis from some diseases much less common.  These diseases include measles, mumps, rubella (German measles), polio, varicella (chicken pox), and pertussis (whooping cough).  There are also vaccines for rabies, bacterial meningitis, Japanese encephalitis, Human Papillomavirus (HPV), and some other diseases that can cause encephalitis.

People can protect themselves from diseases that are spread by insects, like Lyme disease and malaria, by avoiding insect bites.  For example, they can use bug spray; wear long sleeves and long pants; sleep under a mosquito net; and make sure they have good window and door screens.

References

Diseases and disorders of the brain