By way of example, let's turn to marbles. If chips on marbles truly occur randomly, there's no way to conclude anything specific about a certain marble that has a chip, except that marbles get chipped. Now, say that the 100 marbles in the pic represent 100 people. If you know that diseases like the flu, for example, infects birds, pigs, and people, but that only people can die from infection, then you can reasonably say that the flu is only deadly to people. But scientists don't just stop there! They will test that hypothesis further by gathering data from testing populations of pigs and birds that are infected with the flu. If neither pigs nor birds die, this is further evidence that supports their hypothesis, "Only people are susceptible to dying from the flu."
Now let's go back to the marbles as being marbles. If we find that every chipped marble is also one specific color, what do you think we can deduce from that?