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There are over 65,000 known species of protozoa. About 10,000 species are parasitic.
Astigmatism is the most common vision problem. It may accompany nearsightedness or farsightedness. It is usually caused by an irregularly shaped cornea, but sometimes it is the result of an irregularly shaped lens. Either type can be corrected by eyeglasses, contact lenses, or refractive surgery.
Cancer has been around as long as humankind, but only in the second half of the twentieth century did the number of cancer cases explode.
The heart is located in the center of the chest, with part of it tipped slightly so that it taps against the left side of the chest.
The most common childhood diseases include croup, chickenpox, ear infections, flu, pneumonia, ringworm, respiratory syncytial virus, scabies, head lice, and asthma.