A24-year-old nulliparous woman comes into the office because she has not had her menses for six months. she is in good health and not taking any medications. she is not sexually active. she does well in graduate school, despite her demanding new program. her height is 5 feet 6 inches and her weight is 104 pounds. her vital signs are normal. her physical examination, including a pelvic examination, is completely normal. what is the most likely reason for her amenorrhea?

parasites can detrimentally affect their host, but also must not allow their host to die before they can complete their life cycle. some parasites have complex life cycles with many different hosts, and have had to find ways of persisting in many different environments during the change from one host to another. ectoparasites live outside the body of the host, and can more easily  disperse, but are  more exposed to predators, parasites, parasitoids. endoparasites live inside the body of the host, and are therefore protected from the outside environment. but they can be  attacked by the host's immune system. they have therefore developed  mechanisms for dispersal such as  complex life cycles.

desert perennials often survive by remaining dormant during dry periods of the year, then springing to life when water becomes available. most annual desert plants germinate only after heavy seasonal rain, then complete their reproductive cycle very quickly.