Despite all of the preparation, when they approached the ICU wards, feelings of fear and trepidation inevitably began to rise.
"We followed the medical workers and passed through one gate after another to enter the 'red area' (wards for critical patients)," Cao recalls. "Frankly speaking, it felt like we were moving toward the front door of Hades.
"However, when you actually enter the area and see the constant hustle and bustle, and hear the buzz of medical instruments, you understand that it is a battlefield."
Kang Chengye, director of the documentary, has entered the "red areas" five times.
"Here, relations among people are extraordinarily harmonious," Kang says. "No one complains or loses their temper. Patients take every opportunity to show gratitude. Medical workers write words of blessing and their nicknames on their protective suits. Once they leave the wards, they cheer up and tease or joke with each other."