This is because worms do not produce eggs all the time, so even when worms are present a particular stool sample may not contain any eggs. (birdneckanimalhospital.com)
This method of sexual reproduction produces eggs with an extra shell layer that lets them survive the stressful period. (sciencemag.org)
Female fleas start producing eggs within 24 to 48 hours after taking their first blood meal and can lay up to 40 or 50 eggs per day. (lowchensaustralia.com)