Nilotpal Chakravarti in Medium:
Every epidemic, including Covid-19, turns around a single equation,
R = N x p x (1-f).
N is the average number of persons who come into contact with an infected person while she is infectious;
p is the probability of the virus being transmitted during a contact;
f is the fraction of the population immune to the infection.
When an infected person comes into contact with an uninfected person she spreads the virus to the contact with probability p; the contact may either be immune or susceptible. The probability of being immune is f while that of being susceptible is1-f. So p x (1-f) is the probability of the contact becoming infected.
R — the Reproduction Number — is the average number of persons an infected person infects in turn. The disease grows exponentially if R is greater than 1; decays exponentially and dies away if R is less than 1; and persists at a steady rate if R is exactly 1.
The epidemic equation severely restricts the tactics that may be used against the Sars-CoV-2 or any other virus for which no cure or vaccine is available just as the rules of chess dictate how the game may be played.