Dengue fever is an infectious disease of humans that is caused by a virus. There are four main viral strains, and an individual can never be infected by the same strain twice. However, a second infection can occur by any of the other strains. The severity of the second infection depends on the identity of both the strain that caused the first infection and the strain causing the second infection, as well as the order in which these were acquired. Required:
How many different levels of severity are possible for an individual acquiring a second infection

12

Explanation:

Suppose the four different strains are P, Q, R, S. Thus, the first infection can be from any of the four strains and it occurs in 4 ways.

Since an individual cannot be infected again with the strain from which he/she is been previously infected;

Then, at the second time;

The possible number of strain to be infected with = 4-1 = 3 ways

Thus; the number of possible different levels of severity can be viewed by forming two lettered word  P, Q, R, and S without repetition which is:

= 4 × 3

 = 12

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explanation:

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