Humans are similar to other species in many ways, and yet are unique among Earth’s life forms.
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Explain that the human body has ways to defend itself against disease-causing organisms and describe how defenders, including tears, saliva, the skin, some blood cells and stomach secretions support the defense process.
Recognize that there are some diseases that human beings can only get once; and explain how many diseases can be prevented by vaccination.
Explain how vaccines induce the body to build immunity to a disease without actually causing the disease itself.
Recognize a healthy body cannot fight all germs that invade it; and explain how some germs interfere with the body’s defenses.