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When blood is exposed to air, it clots. Heparin allows the blood to come in direct contact with air without clotting.
Vaccines cause herd immunity. If the majority of people in a community have been vaccinated against a disease, an unvaccinated person is less likely to get the disease since others are less likely to become sick from it and spread the disease.
Acute bronchitis is an inflammation of the breathing tubes (bronchi), which causes increased mucus production and other changes. It is usually caused by bacteria or viruses, can be serious in people who have pulmonary or cardiac diseases, and can lead to pneumonia.
Blood in the urine can be a sign of a kidney stone, glomerulonephritis, or other kidney problems.
Cytomegalovirus affects nearly the same amount of newborns every year as Down syndrome.

