Kansen 2: Inzai Toshi(2010)

This is a continuation of Kansen's story: Inyoku no Rensa The story where Hiroshi, Yuuki, Hayami and his school friends are doing a Study Tour. Suddenly while on the way there was a big explosion that destroyed all the cities. At that time the spread of the virus occurred very quickly through sex. How do they survive in this sex virus incident?