You have to make up all prayers you missed during your illness. It was narrated that Anas said: The Messenger of Allah (peace be upon him) said: “Whoever forgets a prayer, let him pray it when he remembers it.”  (An-Nasa`i)

According to the version of Muslim, here is no explanation for it, except this. Qatada said: (Allah says): “And observe prayer for remembrance of Me” (Muslim)

Prayer does not drop even if a person is on the verge of death. If a person cannot pray standing, he/she can pray while sitting or lying down or at any position possible.

Even at time of war while Muslims are engaged in fighting, they have to offer prayers in congregation.

It was narrated from `Abdullah ibn Buraidah that his father said: “The Messenger of Allah (peace be upon him) said: ‘The covenant that stands between us and them is the Salah; whoever abandons it, he has committed disbelief.'” (An-Nasa’i)

Allah Almighty knows best.