context: Chickenpox is a common illness caused by the varicella-zoster virus. Symptoms of chickenpox include fever and itchy spots or blisters all over the body. Chickenpox is usually mild and runs its course in five to 10 days, but it can cause more serious problems when teens and adults get it. People with weakened immune systems are especially susceptible to developing serious complications from chickenpox. Some complications that can arise from chickenpox include: Skin infections Pneumonia Encephalitis (swelling in the brain) Shingles (later in life) Joint inflammation Vaccination is the best way to prevent chickenpox. A chickenpox vaccine has been available in the U. S. since 1995 and is easy to get from a doctor or a public health clinic. The chickenpox vaccine is very effective at preventing the disease -- between 70% and 90% of people who get vaccinated will be completely immune to chickenpox. If a vaccinated person does get chickenpox, the symptoms will be very mild and only last for a few days. All adults who have never had chickenpox or received the vaccination should be vaccinated against it. Two doses of the vaccine should be given at least four weeks apart. If you've never had chickenpox or been vaccinated and you are exposed to chickenpox, being vaccinated right away will greatly reduce your risk of getting sick. Studies have shown that vaccination within three days of exposure is 90% effective at preventing illness; vaccination within five days of exposure is 70% effective. If you do get sick, the symptoms will be milder and shorter in duration. You should not be vaccinated against chickenpox if you: Are moderately to severely ill at the time of vaccination Are pregnant (women should not become pregnant for one month after receiving the chickenpox vaccine) Have ever had an allergic reaction to gelatin, the antibiotic neomycin, or a previous dose of chickenpox vaccine These people should check with their doctor about getting the chickenpox vaccine: Patients undergoing chemotherapy or radiation for cancer People taking steroid drugs People with HIV or another disease that compromises the immune system Patients who recently had a blood transfusion or received other blood products The chickenpox vaccine is made from a live, weakened form of the varicella virus. That means the virus is able to produce immunity in the body without causing illness. The most common side effect from the chickenpox vaccine is swelling, soreness, or redness at the site of the injection. A small number of people may also develop a mild rash or a low-grade fever after vaccination. Serious reactions to the chickenpox vaccine are extremely rare, but they may include: Seizures Brain infection Pneumonia Loss of balance Severe allergic reaction (anaphylaxis) If you think you may be having a serious reaction to the chickenpox vaccine, call your health care provider right away. Make a note of the symptoms you're experiencing, and report them to the Vaccine Adverse Event Reporting System ( VAERS) at 1-800-822-7967. Women who receive the chickenpox vaccine during pregnancy should contact their health care provider right away. Chickenpox during pregnancy can cause birth defects, so there may be a risk that the chickenpox vaccine could cause the same birth defects. As with other vaccines, the risks associated with the chickenpox vaccine are much lower than the risks associated with the disease itself.