In nature a minnow or any fish almost always has one or two parasites, external and/or internal, but in nature those few parasites are probably a simple irritant, and if they become more, that minnow will eventually be dead.
And in nature a single parasite usually doesn't get out of control because in millions of gallons, there are a multitude of hosts to latch onto, but its spread around among the many.
If you put a fish in a tank, or even an average pond, there are a limited number of hosts, in a limited number of recirculating gallons.
This is why in a tank, ick can become a devastating disease in a short time, as its progeny keep on reinfecting the same fish over and over, becoming epidemic, while in a lake, its not more than an irritant. Each ick spot can produce hundreds of new ick, with no other fish to latch onto except the same ones over and over.
I have wild caught fish at the moment, the first day I got them, I noticed one or two, had a single ick parasite, one or two had a few lernaea parasites.
If I wouldn't have treated, it may have only been a few weeks and every fish in the tank would have been infected.

With the Lernaea on this wild caught tetra above, it was pretty easy to see the parasite hanging on the caudal area, but beside treating the tank for juvie Lernaea, I had to physically remove the adult vectors.

Many other parasites are not easily seen, the internal or gill parasites are often invisible and get out of control before being noticed, as would be one or 2 ick spots, that eventually flood the tank with hundreds of progeny.