T:Total population
S:Susceptible (healthy individuals who do not carry the disease, but who could be infected)
I:Infected (individuals who carry the disease and can transmit it)
A:Recovered (individuals who heal and become immunized)
D:Deceased (individuals who have died)
1/i:Average interaction between individuals
1/γ:Average infection time (for a single individual)
β:Rate of infection (probability of a person becoming ill when in contact with an infected person)
r:Average recovery rate (for a single individual)