Z Virus

Probability Level 1

The world had been harmed by a widespread Z-virus, which already turned 10% of the world's population into zombies.

The scientists then invented a test kit with the sensitivity of 90% and specificity of 70%: 90% of the infected people will be tested positive while 70% of the non-infected will be tested negative.

If the test kit showed a positive result, what would be the probability that the tested subject was truly zombie?

If the solution is in a form of $\frac{a}{b}$ , where $a$ and $b$ are coprime positive integers, submit your answer as $a+b$ .