Nancy took a breath and gathered her thoughts. She imagined that passing through the gray-zone door into Level 4 was like a space walk, except that instead of going into outer space, you went into inner space, which was full of the pressure of life trying to get inside your suit. People went into Level 4 areas all the time at the Institute, particularly the civilian animal caretakers. But going into a containment zone to perform a necropsy on an animal that had died of an amplified unknown hot agent was something a little different. This was high-hazard work. –The Hot Zone,
Richard Preston

What is the central idea of this excerpt from The Hot Zone?

Animal necropsies are done only by trained professionals.
People go into Level 4 areas all the time at the Institute.
Entering Level 4 is like going into inner space.
Working in a containment zone is very dangerous.