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Giardia is one of the most common intestinal parasites worldwide, and infects up to 20% of the world population, mostly in poorer countries with inadequate sanitation. Infections are most common in children, though chronic Giardia is more common in adults.
It is believed that humans initially contracted crabs from gorillas about 3 million years ago from either sleeping in gorilla nests or eating the apes.
There are over 65,000 known species of protozoa. About 10,000 species are parasitic.
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.
The most common childhood diseases include croup, chickenpox, ear infections, flu, pneumonia, ringworm, respiratory syncytial virus, scabies, head lice, and asthma.