I believe the answer is...
Bioconversion
Answer: The Digestive System
Explanation: Polio first enters the body through the mouth, usually through contact between people. The infected person can spread it through their nasal and oral secretions (mucous and saliva) as well as through their infected contaminated fecal matter (stool from bowel movement), in cases of lack of hygiene. Once the polio virus is in the digestive tract, it multiplies quickly, then enters the blood, and ends up in the nervous system. From there, it attacks the spine leading to paralysis.
The things that birds have attached to their bodies called their wings, a left one and a right one, cause them to have the ability to fly.
Answer:
seems you forgot the "followings" but here are some known limiting factors
Explanation:
Exercise,Physiology, Mindset, Nutrition and Genetics.