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A once commonly used preservative in multi-dose vaccine vials, thimerosal breaks down into ethylmercury and is excreted from the body. Thimerosal has been used in vaccines, other drugs, and even contact lens solutions since the 1930s. Today, there are no…


A disease caused by a bacterium found in the environment (like soil and manure). The infection causes muscle spasms and stiffness, which can last for weeks. Breathing can be affected by these symptoms. 10-20% of people with tetanus will die,…

Temporal association

Two or more events that occur around the same time but might be unrelated, chance occurrences. For example, a child receives an MMR vaccine around 1 year old - the same time most children start walking.  But the MMR vaccine…


When you are at risk of getting a disease because you are not protected against it.

Sudden Infant Death Syndrome (SIDS)

The sudden and unexpected death of a healthy infant under 1 year of age. A diagnosis of SIDS is made when an autopsy cannot determine another cause of death. The cause of SIDS is unknown. SIDS cases have dropped since…

Subclinical infection

When a person is infected with germs but has no symptoms. It is important to know that someone with subclinical infection may still pass the germs to others. Also known as asymptomatic infection.


A specific version of an organism. Many viruses and bacteria, including influenza, polio, and pneumococcal bacteria, have multiple strains. Scientists can make vaccines that fight multiple strains of the same organism.


An acute, highly contagious, often fatal viral disease and characterized by high fever and aches with subsequent widespread eruption of pimples that blister, produce pus, and form pockmarks. Smallpox has killed millions of people but has not made anyone in…

Side effect

A response that happens as a result of a vaccine, such as redness or swelling at the injection site. Most vaccine side effects are expected and will go away on their own in a day or two.


Viral shedding happens when a virus replicates inside your body and is released into the environment. You may not have symptoms but can still be contagious. Live virus vaccines can also shed the vaccine virus but vaccine shedding is unlikely…

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