There is a relatively new virus called Hosta Virus X (HVX) that has been showing up in nurseries for the last few years. It can be hard to detect because of all of the variations that have been bred into Hosta leaves.
I have read somewhere that the viral discoloration looks the way ink bleeds on fabric- that's a pretty accurate description.
Hosta Virus X does not kill a plant the way a blight or root rot would. Instead, it slowly weakens its immune system, making it less vigorous than other plants. Each year the diseased Hosta will produce smaller leaves and less flowers until it dies.
If your Hostas are infected with HVX, remove them and do not propagate from them. If you are a nursery grower, make sure that any divisions or plants you purchase have been virus-indexed.
This virus spreads on pruning shears and through vegetative propagation (division). When the knife cuts through the fleshy root, it picks up the virus. When a cut is made on another plant, the virus is transferred off the knife and into the plant.
The virus that causes HVX will only infect Hostas.
There are many viruses that infect plants- and many of them create beautiful patterns on the foliage (in a pathologist's eyes):
Tomato Spotted Wilt Virus on Hosta
Nandina Mosaic Virus on 'Firepower' Nandina
Camellia Mottle Virus on Japanese Camellia
Canna Yellow Mottle Virus on Canna Lily
Just like in humans, there is no "cure" for a virus. Therefore, the best way to manage this disease is to dig the infected plants out of your garden or cull them from your nursery stock.