The scientists fall prey to it slowly, one by one. There's no available vaccine or antibiotic and no escape. They wait to die. When only three of the seven scientists remain, one being the medical doctor, Shukaido, he develops a vaccine just as he, himself contracts the virus. Lying to Shion by telling him that both of them will receive the vaccine, he injects only Shion.
Reincarnation comes into play now.
Mokuren forsaw this and before any died, informed them they will meet again, on Earth, as Terrans someday. Shortly after Shukaido dies, Mokuren contracts the disease. She'll await him in a new one. And so, for nine years after, Shion remains alive with only corpses of his friends for company. Torn from his true love and sole happiness and confined to this living hell, Shion goes insane.