The flea lives on the body of its host and feeds on its blood. why is it in the best interest of the flea not to kill its host?

If the flea were to kill the host, it wouls have to do extra work to find another food source because it just eliminated the food source or host it had previously found. To clarify, it is not in its best interest to kill the host because it wants/needs to continue to use it as a food source instesd of having to find another one.

People who utilizes the progression principle will ensure their exercise program increases in frequency, intensity, time, and type. the changes need to happen gradually in order to ensure they make gains in their fitness program. a person who doesn't utilize the progression principle can still gain health benefits from exercising, but will not continue to improve in cardiovascular fitness.