5, 4, 3, 2, 1 Counting Down!

A boy, Shironi who suffered mysophobia for a long time suddenly meet Kusero from an accident. On their first meeting Kusero can tell right away that Shironi is mysophobic and suggest him to consult a doctor before it gets worst. Shironi was left perturbed by the encounter. Who is Kusero? Why did he tell him to find a doctor? And how was he able to tell Shironi's condition ?