ok, if it is the second case:

given:
* 100 infected people take the test, 95% turn out HIV阳性. and 100 uninfected people take the test, 5% turn out HIV阳性.
* actual infection rate is 0.1%

if 1,000,000 people take the test, 1,000 of them are infected. but the test result will say 50,950 are infected:
1,000,000 * 0.05 + 1,000 * 0.95 = 50,950

so the 小王感染的概率 is 950/50,950 = 2%