Why is there no obvious reason given for the spread of the "milky whiteness?"

When the woman talks to her husband about his condition, she realizes that his blindness is even stranger than the other cases, in that, instead of seeing nothing, he sees a milky whiteness everywhere he looks. The "milky whiteness" would reflect what he is seeing instead of the world around him. It seems to be more of a spreading, the whiteness might signal the infection itself, or a change is the cause.