Encephalitis is inflammation (swelling) of the brain. It can happen when an infection or medical condition that affects the brain activates your immune system. The inflammation can cause a wide range of symptoms. In extreme cases, it can lead to brain damage, stroke, or even death.

There are different types of encephalitis, based on what the cause is. The two main types are infectious encephalitis and autoimmune encephalitis.

Infectious encephalitis is usually caused by a virus. In fact, viruses are the most common cause of encephalitis. Some of the different viruses that cause it include:

• Herpes viruses, including herpes simplex (HSV), the Epstein-Barr virus (which causes infectious mononucleosis) and the varicella-zoster virus (which causes chickenpox).
• Viruses you can get if you are bitten by an infected tick, such as tick-borne encephalitis (TBE virus) and Powassan virus.
• Viruses you can get if you are bitten by an infected mosquito, such as eastern equine encephalitis virus, West Nile virus, and La Crosse virus.
• Enteroviruses, which are a common group of viruses that mostly cause mild illness or respiratory infection. These infections usually happen in the summer and fall.

Bacteria, fungi, and parasites can also cause infectious encephalitis. But this is not common.

Autoimmune encephalitis happens when your immune system mistakenly attacks healthy brain cells. It can be triggered by conditions such as certain cancers, benign tumors, and infections. Sometimes the cause is not known.

Anyone can get encephalitis, but you are more likely to get it if you:

• Have a weakened immune system, for example from having HIV or taking certain medicines. These could include medicines taken after an organ transplant, certain chemotherapy medicines, and specialized treatments for certain autoimmune diseases.
• Are a young child or older adult.
• Live in areas where there are ticks and mosquitoes that carry viruses that can cause encephalitis.

The symptoms of encephalitis can vary a lot, depending on how severe it is. Many people do not have any symptoms. Others may have mild flu-like symptoms such as fever, fatigue, headache, or body aches. If encephalitis becomes more serious, it can cause:

• Severe headache
• Stiff neck
• Vomiting
• Seizures
• Behavior changes
• Drowsiness
• Muscle weakness
• Partial paralysis in your arms and legs
• Coma

Encephalitis can be dangerous in infants. Their symptoms may include:

• Fever
• Lethargy (weakness or drowsiness)
• Poor feeding
• Vomiting
• Body stiffness
• Unusual irritability or crying
• A full or bulging fontanel (the soft spot on the top of the head)

If you or your child is having symptoms of encephalitis, it's important to get medical care right away.

To find out if you have encephalitis, your health care provider:

• Will do a physical exam
• Will take your medical history, which includes asking about your symptoms
• May do a neurologic exam
• May order imaging tests, such as a brain CT scan or MRI
• May order an EEG (electroencephalography), which use small electric sensors to measure your brain activity
• May order blood and cerebrospinal fluid (CSF) tests

Most people with encephalitis will need treatment in the hospital. Depending on the cause, treatments may include antiviral medicines, antibiotics, corticosteroids, and other medicines.

For some types of encephalitis, there is no medicine to treat it. But rest, nutrition, and fluids can help your body fight the infection and relieve symptoms.

Some people may need physical, speech, and occupational therapy once the illness is under control.

There are steps you can take to help prevent encephalitis that is caused by infections:

• Use good hygiene, including washing your hands often with soap and water.
• Don't share food, drinks, utensils, and glasses with other people.
• Get vaccines for viruses that can cause encephalitis.
• Avoid mosquito and tick bites, for example by:
  • Wearing insect repellent with DEET or another U.S. Environmental Protection Agency (EPA)-registered insect repellent. Make sure to follow the instructions for using the repellant.
  • Wearing clothes that cover your arms, legs and feet.
  • Treating your clothing and gear with products containing 0.5% permethrin before you go in grassy or woody areas.

NIH: National Institute of Neurological Disorders and Stroke

Germs are microorganisms. This means that they can be seen only through a microscope. They can be found everywhere - in the air, soil, and water. There are also germs on your skin and in your body. Many germs live in and on our bodies without causing harm. Some even help us to stay healthy. But some germs can make you sick. Infectious diseases are diseases that are caused by germs.

The main types of germs are bacteria, viruses, fungi, and parasites.

There are different ways that germs can spread, including:

• Through touching a person who has the germs or making other close contact with them, such as kissing, hugging, or sharing cups or eating utensils
• Through breathing air after a person with the germs coughs or sneezes
• Through touching the feces (poop) of someone who has the germs, such as changing diapers, then touching your eyes, nose, or mouth
• Through touching objects and surfaces that have germs on them, then touching your eyes, nose, or mouth
• From mother to baby during pregnancy and/or childbirth
• From insect or animal bites
• From contaminated food, water, soil, or plants

You can help protect yourself and others from germs:

• When you have to cough or sneeze, cover your mouth and nose with a tissue or use the inside of your elbow
• Wash your hands well and often. You should scrub them for at least 20 seconds. It is important to do this when you are most likely to get and spread germs:
  • Before, during, and after preparing food
  • Before eating food
  • Before and after caring for someone at home who is sick with vomiting or diarrhea
  • Before and after treating a cut or wound
  • After using the toilet
  • After changing diapers or cleaning up a child who has used the toilet
  • After blowing your nose, coughing, or sneezing
  • After touching an animal, animal feed, or animal waste
  • After handling pet food or pet treats
  • After touching garbage
• If soap and water are not available, you can use an alcohol-based hand sanitizer that contains at least 60% alcohol
• Stay home if you are sick
• Avoid close contact with people who are sick
• Practice food safety when handling, cooking, and storing food
• Regularly clean and disinfect frequently touched surfaces and objects

Giant cell arteritis is a disorder that causes inflammation of your arteries, usually in the scalp, neck, and arms. It narrows the arteries, which keeps blood from flowing well. Giant cell arteritis often occurs with another disorder called polymyalgia rheumatica. Both are more common in women than in men. They almost always affect people over the age of 50.

Early symptoms of giant cell arteritis resemble the flu: fatigue, loss of appetite, and fever. Other symptoms include:

• Headaches
• Pain and tenderness over the temples
• Double vision or visual loss, dizziness
• Problems with coordination and balance
• Pain in your jaw and tongue

Your doctor will make the diagnosis based on your medical history, symptoms, and a physical exam. There is no specific test for giant cell arteritis, but you may have tests that measure inflammation.

Treatment is usually with corticosteroids. Early treatment is important; otherwise there is a risk of permanent vision loss or stroke. However, when properly treated, giant cell arteritis rarely comes back.

NIH: National Institute of Arthritis and Musculoskeletal and Skin Diseases

HIV stands for human immunodeficiency virus. HIV harms your immune system by destroying a type of white blood cell that helps your body fight infection. This puts you at risk for other infections and diseases.

AIDS stands for acquired immunodeficiency syndrome. It is the final stage of infection with HIV. It happens when the body's immune system is badly damaged because of the virus. Not everyone with HIV develops AIDS.

HIV is spread through certain body fluids from a person who has HIV. This can happen:

• By having unprotected vaginal or anal sex with a person who has HIV. "Unprotected" means not using condoms or medicine to treat or prevent HIV. This is the most common way that it spreads.
• By sharing drug needles.
• Through contact with the blood of a person with HIV.
• From mother to fetus during pregnancy.
• From mother to baby during, childbirth, or breastfeeding.

Anyone can get HIV, but certain groups have a higher risk of getting it:

• People who have another sexually transmitted infection (STI). Having an STI can increase your risk of getting or spreading HIV.
• People who inject drugs with shared needles.
• Gay and bisexual men.
• Black/African Americans and Hispanic/Latino Americans. They make up a higher proportion of new HIV diagnoses and people with HIV, compared to other races and ethnicities.
• People who engage in risky sexual behaviors, such as not using condoms or medicine to prevent or treat HIV.

Factors such as stigma, discrimination, income, education, and geographic region can also affect people's risk for HIV.

The first signs of HIV infection may be flu-like symptoms:

• Fever
• Chills
• Rash
• Night sweats (heavy sweating during sleep)
• Muscle aches
• Sore throat
• Fatigue
• Swollen lymph nodes
• Mouth ulcers

These symptoms may come and go within two to four weeks. This stage is called acute HIV infection.

If the infection is not treated, it becomes chronic HIV infection. Often, there are no symptoms during this stage. If it is not treated, eventually the virus will weaken your body's immune system. Then the infection will progress to AIDS. This is the late stage of HIV infection. Because your immune system is badly damaged, your body cannot fight off other infections, called opportunistic infections (OIs). OIs are infections that happen more frequently or are more severe in people who have weakened immune systems.

Some people may not feel sick during the earlier stages of HIV infection. So the only way to know for sure whether you have HIV is to get tested.

A blood test can tell if you have HIV infection. Your health care provider can do the test, or you can use a home testing kit. You can also use the CDC Testing Locator to find free testing sites.

There is no cure for HIV infection, but it can be treated with medicines. This is called antiretroviral therapy (ART). ART can make HIV infection a manageable chronic condition. It also reduces the risk of spreading the virus to others.

Most people with HIV live long and healthy lives if they get ART as soon as possible and stay on it. It's also important to take care of yourself. Making sure that you have the support you need, living a healthy lifestyle, and getting regular medical care can help you enjoy a better quality of life.

You can reduce the risk of getting or spreading HIV by:

• Getting tested for HIV.
• Choosing less risky sexual behaviors. This includes limiting the number of sexual partners you have and using latex condoms every time you have sex. If your or your partner is allergic to latex, you can use polyurethane condoms.
• Getting tested and treated for sexually transmitted infections (STIs).
• Not injecting drugs.
• Talking to your provider about medicines to prevent HIV:
  • PrEP (pre-exposure prophylaxis) is for people who don't already have HIV but are at very high risk of getting it. PrEP is daily medicine that can reduce this risk.
  • PEP (post-exposure prophylaxis) is for people who have possibly been exposed to HIV. It is only for emergency situations. PEP must be started within 72 hours after a possible exposure to HIV.

NIH: National Institutes of Health

Your immune system is a complex network of cells, tissues, and organs. Together they help the body fight infections and other diseases.

When germs such as bacteria or viruses invade your body, they attack and multiply. This is called an infection. The infection causes the disease that makes you sick. Your immune system protects you from the disease by fighting off the germs.

The immune system has many different parts, including:

• Your skin, which can help prevent germs from getting into the body
• Mucous membranes, which are the moist, inner linings of some organs and body cavities. They make mucus and other substances which can trap and fight germs.
• White blood cells, which fight germs
• Organs and tissues of the lymph system, such as the thymus, spleen, tonsils, lymph nodes, lymph vessels, and bone marrow. They produce, store, and carry white blood cells.

Your immune system defends your body against substances it sees as harmful or foreign. These substances are called antigens. They may be germs such as bacteria and viruses. They might be chemicals or toxins. They could also be cells that are damaged from things like cancer or sunburn.

When your immune system recognizes an antigen, it attacks it. This is called an immune response. Part of this response is to make antibodies. Antibodies are proteins that work to attack, weaken, and destroy antigens. Your body also makes other cells to fight the antigen.

Afterwards, your immune system remembers the antigen. If it sees the antigen again, it can recognize it. It will quickly send out the right antibodies, so in most cases, you don't get sick. This protection against a certain disease is called immunity.

There are three different types of immunity:

• Innate immunity is the protection that you are born with. It is your body's first line of defense. It includes barriers such as the skin and mucous membranes. They keep harmful substances from entering the body. It also includes some cells and chemicals which can attack foreign substances.
• Active immunity, also called adaptive immunity, develops when you are infected with or vaccinated against a foreign substance. Active immunity is usually long-lasting. For many diseases, it can last your entire life.
• Passive immunity happens when you receive antibodies to a disease instead of making them through your own immune system. For example, newborn babies have antibodies from their mothers. People can also get passive immunity through blood products that contain antibodies. This kind of immunity gives you protection right away. But it only lasts a few weeks or months.

Sometimes a person may have an immune response even though there is no real threat. This can lead to problems such as allergies, asthma, and autoimmune diseases. If you have an autoimmune disease, your immune system attacks healthy cells in your body by mistake.

Other immune system problems happen when your immune system does not work correctly. These problems include immunodeficiency diseases. If you have an immunodeficiency disease, you get sick more often. Your infections may last longer and can be more serious and harder to treat. They are often genetic disorders.

There are other diseases that can affect your immune system. For example, HIV is a virus that harms your immune system by destroying your white blood cells. If HIV is not treated, it can lead to AIDS (acquired immunodeficiency syndrome). People with AIDS have badly damaged immune systems. They get an increasing number of severe illnesses.