Question: Differential equation of an Epidemic

Hi,

I need to find the differential equation of the following situation:

There's is an epidemic in a city of a certain poupulation(P). The rate of the spread is propoptionnal to the to the number of people infected times the number of person who are not infected.

I need to find the differential equation of this situation where N(t) will represent the number of people infetected(N) bas on the time. The only information I have is the populaiont(P), the porpotionnality constant (K) and the inital proporion of people who are infected.

Here are the value: P=75000 , K=0.02 and PO=0.02

I know that the differetial equation willhave this form : dN/dT=.... But I dont know the rest

Can you help me please? I tried a lot of things but nothing seem to work

Thanks, Jama