Definitely a virus, but which one is nearly impossible to tell without lab testing. Squash mosaic virus is quite common in squash, can be seed transmitted or introduced by cucumber beetles, and can spread plant-to-plant by handling/brushing leaves. It could be CMV as described earlier; others such as watermelon mosaic virus, papaya ringspot virus, or zucchini yellow mosaic virus are also found in cucurbits. Sometimes the fruit have no visible symptoms (not discolored, bumpy, knobby, etc.) - they are safe to eat. Treatment is pretty much the same for these viruses. They can spread to your other cucurbits - cucumbers, melons - so you have to make the choice of keeping or removing the plant(s).