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The most destructive flu epidemic of all times in recorded history occurred in 1918, with approximately 20 million deaths worldwide.
Amoebae are the simplest type of protozoans, and are characterized by a feeding and dividing trophozoite stage that moves by temporary extensions called pseudopodia or false feet.
Each year in the United States, there are approximately six million pregnancies. This means that at any one time, about 4% of women in the United States are pregnant.
Since 1988, the CDC has reported a 99% reduction in bacterial meningitis caused by Haemophilus influenzae, due to the introduction of the vaccine against it.
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.