A virologist has discovered a virus that has recently been introduced in the human population. It appears that the virus

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A virologist has discovered a virus that has recently beenintroduced in the human population. It appears that the virus isquite harmful. Unfortunately, not much is known
about the ability of the virus to spread within the humanpopulation. The limited evidence suggests that an infected personon average infects two other persons. To
test this hypothesis, the virologist resorts to an animal modelof the infection, using macaques. He experimentally infects onemacaque with the virus, and puts the
infected animal in a cage with two uninfected animals. Toevaluate if the uninfected animals have been infected, blood istaken from the uninfected animals at the end of
the experiment, and checked for antibodies against thepathogen.
(Question): Draw a diagram with on the x-axis the number ofsusceptible individuals (S) and on the y-axis the number ofinfected individuals (I). The nodes (S,I)=(0,0), (S,I)=(0,1),
(S,I)=(0,2), (S,I)=(0,3), (S,I)=(1,0), (S,I)=(1,1), (S,I)=(1,2),(S,I)=(2,0), and (S,I)=(2,1) denote the possible states of theexperimental epidemic. Draw arrows for all possible transitionsbetween states.