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# Differential equations to describe infection

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Use models to describe the population dynamics of disease agents.

Total population is a Constant (T).
A small group of infected individuals are introduced into a large population. Describe spread of infection within population as a function of time. This disease which, after recovery, confers immunity. The population can be divided into three distinct classes.

a) A susceptible class S, who can catch the disease
b) the infected class I - who have the disease and can transmit.
c) the removed class R, who have had the disease or are recovered, immune or isolated until recovered, or indeed, dead.

The gain in the infected class is at a rate proportional to the number of infected and susceptible persons that is rSI, where r > 0 (is a constant) r >0 is called the infection rate. The susceptibles are lost at the same rate.

The rate of removal of infected to the removed class is proportional to the number of infected people, that is aI, where a > 0 is a constant. a >0 is called the removal rate.

The incubation period is short enough to be negligible.

S(0) = So>0, I(0) = Io>), R(0) =0

#1 Formulate the three differential equations describing the rate of change in the number of S, I, and R populations.

A key questions is that given r, a, and S and the initial number of infected individuals (Io) whether the infection will spread or not, and if it does how it develops with time and of course when it will start to decline. The term epidemic means that I(t) >Io for some t > 0

#2 Find the minimum number of suseptibles required to start an epidemic in terms of a and r.

Assume that a=0.5, r=0.005, So = 150 Io = 50, N = 200

#3 Is this epidemic probable or not?

#4 Determine the global maximum number of infected population in this disease process

#5 When does the maximum in the infected class happen?

##### Solution Summary

The solution answers the question(s) below.

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###### Education
• BSc , Wuhan Univ. China
• MA, Shandong Univ.
###### Recent Feedback
• "Your solution, looks excellent. I recognize things from previous chapters. I have seen the standard deviation formula you used to get 5.154. I do understand the Central Limit Theorem needs the sample size (n) to be greater than 30, we have 100. I do understand the sample mean(s) of the population will follow a normal distribution, and that CLT states the sample mean of population is the population (mean), we have 143.74. But when and WHY do we use the standard deviation formula where you got 5.154. WHEN & Why use standard deviation of the sample mean. I don't understand, why don't we simply use the "100" I understand that standard deviation is the square root of variance. I do understand that the variance is the square of the differences of each sample data value minus the mean. But somehow, why not use 100, why use standard deviation of sample mean? Please help explain."
• "excellent work"
• "Thank you so much for all of your help!!! I will be posting another assignment. Please let me know (once posted), if the credits I'm offering is enough or you ! Thanks again!"
• "Thank you"
• "Thank you very much for your valuable time and assistance!"

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