I also agree with Rowan, these days we are so hyper clean that we no longer have any resistance to bugs that used to be harmless.
I remember as children being brought up in the countryside if we were out and about and got thirsty we would think nothing of drinking from the stream that ran through all the farms and fields, heaven knows what contaminated this water as the cows and the sheep waded through it, it was fresh, tasty and quenched our thirst, these days you would be rushed to A&E at the mere thought of drinking it. It never done us any harm.