For the probability of the number of infective's increasing in one time step, I found the answer is:(adsbygoogle = window.adsbygoogle || []).push({});

∆tβi(N-i)/N

where β is the contact rate, ∆t is the time step, i is number of infectives,N is total number of susceptible's and infective's

I can't quite see where this is coming from. β is the contact rate, so it makes sense to multiply by i since for more i, more chances of encounters. Then multiplying by N-i makes sense since more susceptible also means more chance of encounters.

But then why the division by N? I think my problem may be in understanding what the "contact rate" actually means

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# SIS Epidemic Modelling

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