There is a theory that some genetically programed ich cysts have the ability to lie dormant dried for years, and be carried by the wind, and when they encounter water, have an ability "wake up" so to speak when fish are present. Similar to the way the bacteria that causes ducklips can remain viable in mud.
Fish that have had ich in the past and recover, develop a sort of immunity, and resist infection, but if a new fish is added that doesn't have that immunity becomes infected, the sheer numbers of ich parasites can overwhelm even the seemingly immune fish in the confines of an aquarium.
In nature because of space, an ich parasite is often little more than an irritant, but in tiny closed systems like aquariums or small ponds, epidemic outbreaks can easily occur.
Fish that have had ich in the past and recover, develop a sort of immunity, and resist infection, but if a new fish is added that doesn't have that immunity becomes infected, the sheer numbers of ich parasites can overwhelm even the seemingly immune fish in the confines of an aquarium.
In nature because of space, an ich parasite is often little more than an irritant, but in tiny closed systems like aquariums or small ponds, epidemic outbreaks can easily occur.