Sarraa- It can be lethal if not held in check. It is harder to kill with other tropical fish that cannot withstand the higher water temps. The normal way (in cooler temps) to treat it is with a copper based medication. The life-cycle of the Ich parasite can be as long as 10 days or so. I'm not sure how much you know about the disease, so please forgive me if I'm telling you stuff you already know.
The spots you see are actually the cycst (adult) form of the disease. These spots are actually under the skin of the fish, and there's nothing you can do to "kill" the spots. This is the adult stage of the parasite, and once you see the spots, you know the fish are infected. Once the cysts reach full size, they fall off the fish to the bottom of the tank where they rupture, and spend several hours to days reproducing (by cell division). The smaller parasites (invisible to the eye) will develop tails and begin to actively swim about the tank looking for a new host. It is only when they are in this stage of the life cycle that they can be killed. Once they find a host (your fish) they attach themselves and begin feeding off the fish where they develop into the white spots you see, and they whole cycle begins again.
As I mentioned, water temps play a huge role in the life cycle of the parasites. With water temps in the 60s or 70s, the life cycle can take a couple of weeks. With temps around 85, in can be just a matter of a couple of days. There is a bunch of info out there that states that the free-swimmers cannot live with temps above 85 degrees. Like I said, the best course of action is to raise the temps up to just above 85, and leave it there for a week to 10 days. That should take care of the Ich throughout your entire tank.