Sampling and zero measurement error



I have a sample of 5,000 individuals representing a population 1,000 time larger (meaning 5,000,000).

If I repeat an experiment 100 times on the 5,000 individuals and I obtain always negative response from all the individuals, what is the number I can quote as my measurement?

Is it 0 with a one-sided uncertainty above 0? Or, what can I say from the experiment?

Many thanks!