Suppose that the probability of a person having the fever is $5/100$.
The probability that a person with the fever tests negative is $2/100$.
The probability that a person who does not have the fever tests positive is $10/100$.
If person X tests positive, what is the probability that people have the disease
let $D$ disease, $P$ positive.(using the bayes's theorem)
$$P(D | P) = p(P | D)*p(D)/p(P)$$
$$(5/100) - (2/100) = 3/100. = p(P | D)$$
and $$p(P) = 3/100 + 10/100 = 13/100$$