Encephalitis is a serious inflammatory condition in which the brain becomes swollen. This swelling can affect movement, thinking, mood, behavior, and levels of consciousness. Thousands of people are hospitalized for encephalitis each year, and it remains a medical emergency that can cause long-term disability or even death.

Prompt diagnosis and early treatment are critical. Management depends on the underlying cause but always requires hospitalization, close monitoring, and supportive care.

Types Of Encephalitis

Encephalitis is typically classified based on its root cause.

Infectious encephalitis

This is the most common category. It involves inflammation caused by viruses, bacteria, fungi, or parasites. Each infectious source has different diagnostic features and may require specific treatment.

Autoimmune encephalitis

Autoimmune encephalitis occurs when the immune system becomes abnormally activated and mistakenly attacks the brain. Some experts believe autoimmune forms are more common than previously recognized.

Subtypes of autoimmune encephalitis include:

  • N-methyl-D-aspartate receptor (NMDAR)-antibody encephalitis
    The most well-known subtype, more common in younger adults—especially women. It typically involves abnormal antibodies in cerebrospinal fluid along with signs of inflammation.

  • LGI1-antibody encephalitis
    Often affects older adults, especially men. It rarely shows antibodies in cerebrospinal fluid.

  • Rasmussen’s encephalitis
    A very rare condition that affects a single hemisphere of the brain and usually begins in children ages 6–8.

Cancer-related encephalitis

In rare cases, encephalitis develops as an immune response to cancers such as breast, ovarian, lung cancer, or lymphoma. This is called paraneoplastic encephalitis and is different from cancer that spreads to the brain.

Encephalitis Symptoms

Because the brain controls many essential functions, symptoms depend on which regions are affected. Mental status changes are common and may include confusion, excessive sleepiness, agitation, or behavioral changes. Symptoms usually progress quickly and may lead to coma in severe cases.

Other possible symptoms include:

  • Fever

  • Seizures

  • Muscle weakness

  • Numbness

  • Involuntary movements or tremors

  • Poor coordination

  • Speech or language problems

  • Vision changes

  • Mood or behavioral shifts

  • Memory impairment

Some autoimmune types develop more gradually, producing symptoms that come and go over weeks or months. Specific patterns help identify different subtypes—for example, NMDAR-antibody encephalitis often causes aggression, mood swings, and abnormal movements, while Rasmussen’s encephalitis typically leads to severe seizures and weakness on one side of the body.

Symptoms In Infants

Encephalitis can occur at any age, including infancy. Possible signs in infants include:

  • Low energy

  • Fever

  • Vomiting

  • Not waking up to eat

  • Stiffness

  • Bulging soft spot on the head

Causes Of Encephalitis

Determining the cause is challenging. Even after extensive evaluation, healthcare providers identify the exact cause in only about half of cases.

Some symptoms occur due to direct damage from the infectious organism, while others result from the body’s immune response, which can also contribute to brain inflammation.

Risk factors may include:

  • Exposure to viruses or parasites during travel

  • Living in close quarters such as dormitories

  • A weakened immune system from conditions like HIV, diabetes, or cancer

Infectious causes

Viruses are the most common source.
Examples include:

  • Herpes simplex virus (HSV-1 and HSV-2)

  • Japanese encephalitis virus (common globally, spread by mosquitoes)

  • Arboviruses (transmitted by ticks and mosquitoes)

  • Enteroviruses

  • Adenoviruses

  • Epstein-Barr virus

  • Hepatitis A and B

  • Malaria

  • Varicella-zoster virus (shingles)

  • Rabies

Autoimmune causes

Researchers believe a combination of genetic and environmental triggers may lead the immune system to produce harmful antibodies that target brain tissue.

Cancer-related causes

Certain cancers may trigger an unintended immune reaction that leads to brain inflammation.
These cancers include:

  • Lung cancer

  • Breast cancer

  • Ovarian cancer

  • Lymphoma

Diagnosis

Encephalitis is diagnosed when symptoms last at least 24 hours without another identifiable cause. Many other medical conditions resemble encephalitis and must be ruled out, including:

  • Sepsis

  • Meningitis

  • Vasculitis

  • Low sodium or glucose levels

  • Drug toxicity

  • Brain tumors

  • Stroke

A provider will take a detailed history, perform a physical exam, and ask about symptoms, travel, or tick exposure.

Diagnostic tools include:

  • MRI scans to check for inflammation

  • Cerebrospinal fluid testing via lumbar puncture

  • Blood and urine tests for infection

  • Electroencephalogram (EEG) to detect seizures

  • CT scans if MRI is not available

  • Chest imaging to detect related infections

  • Specific autoantibody tests for autoimmune encephalitis

Encephalitis Treatment

Immediate, aggressive treatment is essential. Care focuses on treating the cause and preventing complications. A neurological team and sometimes an ICU team are typically involved.

Direct treatment of underlying cause

  • Viral encephalitis → antivirals (e.g., acyclovir)

  • Bacterial encephalitis → antibiotics

  • Autoimmune encephalitis → medications that calm the immune system, including:

    • Corticosteroids

    • Immunoglobulin therapy

    • Plasmapheresis

    • Immunosuppressive agents

  • Paraneoplastic encephalitis → tumor removal plus immune-modulating treatment

Supportive treatment

Many patients require additional care, especially in severe cases. Supportive treatment may include:

  • Seizure medications

  • Treatments to reduce brain swelling

  • Intubation or ventilation if breathing becomes impaired

Long-term rehabilitation may involve:

  • Physical therapy

  • Speech therapy

  • Occupational therapy

Prevention

While many cases cannot be prevented, certain steps reduce risk:

  • Frequent handwashing

  • Avoiding shared food or utensils

  • Avoiding unsafe food or water

  • Reducing exposure to insects

  • Staying up to date with childhood and travel vaccines

Complications

Outcomes vary widely. Some individuals fully recover, while others experience permanent neurological impairment affecting:

  • Memory

  • Behavior

  • Motor skills

  • Vision

  • Speech

  • Nerve function

People with autoimmune encephalitis may be at increased risk of developing other autoimmune conditions such as Hashimoto’s disease or possibly lupus.

Living With Encephalitis

Prompt medical treatment can improve outcomes and reduce complications. Around half of individuals with encephalitis experience long-term challenges that may require ongoing therapy, medications, and rehabilitation.

Recovery can be slow and may take months or years. Working closely with a multidisciplinary medical team helps support long-term function and quality of life.