When a predator sees a group of possible preys, the first thing he does is to observe the them to detect the weakest one. Hence the weakest animal has to hide his pain/sickness and pretend to be ok.

In this context, when female mammals give birth, they automatically eat their placenta and lick the blood on the floor: to leave almost no traces that would give clues to predators that birth has taken place (to protect their offspring, in other words)