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There are over 65,000 known species of protozoa. About 10,000 species are parasitic.
Ether was used widely for surgeries but became less popular because of its flammability and its tendency to cause vomiting. In England, it was quickly replaced by chloroform, but this agent caused many deaths and lost popularity.
Signs of depression include feeling sad most of the time for 2 weeks or longer; loss of interest in things normally enjoyed; lack of energy; sleep and appetite disturbances; weight changes; feelings of hopelessness, helplessness, or worthlessness; an inability to make decisions; and thoughts of death and suicide.
The average human gut is home to perhaps 500 to 1,000 different species of bacteria.
Vaccines prevent between 2.5 and 4 million deaths every year.