The first round:
1 person passed it on to X people, and now x+ 1 person has the flu.
Pay attention to who started it.
The second round:
X+ 1 person infects X people, so it will infect (x+ 1) X people in total, so it will be infected.
X(x+ 1) Pay attention to adding your own x+ 1 person.
So now there is a total of x(x+ 1)+x+ 1.
Finishing:
y=x^2+2x+ 1