Diff. EQ. Help - model for disease

The following problem is an SIS disease problem:

Calling: I(t) = number of infectives at time t

N = the total population (assumed constant)

b = infection rate (here, a positive constant)

v = recovery rate (also, a positive constant)

a model for this disease is given bu the following:

dI/dt = bI(N-I) - vI

And since the population is assumed constant, we can just take S(t) to be N -I(t). Derive a condition for when the number of infectives goes to zero.

Is there anyone out there than can help me, even if it's just a little bit?