An individual was exposed to a contagious disease but did not become infected. the agent was unable to enter the body and grow because it was stopped by the person's skin. this is an example of how a can prevent disease.

The answer is body's line of defense. The first line of defense which includes physical as well as chemical barriers would defend the body from infection. It would include the tears, cilia, stomach acid,urine, white blood cells,mucus and the skin. The skin is the largest organ and would act as a barrier from pathogens. If first line of defense won't work the second line will then be activated.

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The person who utilize the progression principle will keep adding the difficulty of the exercise that he's doing, while the person who does not, will only stay stagnant.  the person that utilize progression principle, will get a stronger endurance because now his body is conditioned to do harsher routines.{the answer was already on verified)