If you have a weakened immune system, your body isn't able to fully recognize and eliminate disease-producing bacteria and viruses. This is known as being immunocompromised. Being immunocompromised increases your vulnerability to severe illness and even death.

Since it's usually impossible to avoid exposure to all disease-causing agents, you can decrease your chance of acquiring many serious infections by receiving appropriate and available immunizations.

You can become immunocompromised for a variety of reasons:

• You take medications that intentionally suppress your immune system to prevent rejection of organ or bone marrow transplants
• Radiation therapy
• High doses of corticosteroids
• HIV infection
• Chemotherapy
• Diabetes
• Having your spleen removed or damaged, for instance, from sickle cell anemia
• Advanced age — significant defects in the immune system often occur at approximately age 65 years or older
• Certain malignant diseases, such as lymphoma

If you are immunocompromised, you're more likely to acquire an infectious disease, and the resulting illness may be more severe.

When a healthy person gets the flu (influenza), for instance, his or her immune system works to eliminate the virus. An immunocompromised person may not have white blood cells in adequate numbers or with sufficient ability to identify and eliminate the influenza-infected cells. The virus-infected cells can then multiply, spread throughout the body and cause a life-threatening illness.

Immunizations can partially compensate for an impaired immune system. Recommended vaccines will vary depending on your particular circumstances, but pneumococcal and influenza vaccines are usually two that all immunocompromised people should receive.

Most vaccines contain a weakened or dead disease germ or part of a disease germ. Other vaccines use inactivated toxins. Immunocompromised individuals generally aren't given live viral vaccines, such as measles-mumps-rubella (MMR), chicken pox (varicella) or yellow fever vaccines, because the live viruses may cause complications in immunocompromised people. One exception: Some HIV-positive individuals may be given the measles vaccine, depending on the stage of their illness.

Unfortunately, vaccinations cannot completely make up for an impaired immune system.

"When the average healthy person receives influenza vaccine, he or she is about 70- percent to 90-percent protected against symptoms of influenza, whereas an elderly immunocompromised person might be about 30-percent to 40-percent protected. But if we don't give them the vaccine, it's zero protection, so it does offer something. But it's not quite the same level of protection that you would normally demand of a vaccine," says Gregory Poland, M.D., chairman of Mayo Clinic's Vaccine Research Group, Rochester, Minn.

"Nonetheless, even among elderly immunocompromised individuals, influenza vaccine offers a 50-percent to 80-percent rate of protection against hospitalization and death," says Dr. Poland.

Because vaccines cannot offer complete protection, be sure to take additional steps to stay healthy.

First, remember the basics:

• Get enough sleep.
• Eat a balanced diet.
• Get adequate exercise.

This will help your immune system function up to its ability.

Some additional tips for staying healthy include:

• If you develop a fever or an unusual symptom or acquire an infection, contact your doctor promptly.
• Avoid crowds or wear a mask to help prevent respiratory diseases.
• Avoid putting your hands into your mouth or eyes.
• Wash your hands well and often.
• Consider having your family and caregivers immunized against pneumonia, influenza, MMR and chicken pox if they aren't already immune.

If you travel internationally, you can be exposed to diseases that are rarely, if ever, seen in your home country. You may require either additional vaccines or modifications to the usual travel vaccine schedule. Check with your doctor or a travel medicine clinic before your trip.

It's a good idea to wear a medical alert bracelet or necklace or carry a wallet card stating that you are immunocompromised. "In the event that someone becomes ill or loses consciousness in an accident, it is imperative that health care workers know if that person is immunocompromised," says Dr. Poland.

In some cases, people are immunocompromised only temporarily. For instance, in some arthritic conditions, an individual may receive high doses of corticosteroids for a short time. Although that person will be immunocompromised while on treatment medications, it's not permanent. In such a situation, a doctor may not recommend additional vaccinations.

*Though it is no longer the case in the United States, many countries still give children oral polio vaccine. If you were to come into contact with this immunizing virus, such as from changing the diaper of a child who had received the vaccine, it is possible that the polio virus vaccine could be transmitted. This could occur, for example, if a child has been adopted from overseas and now lives in the United States. To prevent possible transmission in this situation, contact your doctor regarding necessary precautions for interacting with the child in the interval after polio immunization.

Source: Centers for Disease Control Advisory Committee on Immunization Practices