Well, they'd have to be before the big bang, otherwise that wouldn't match current physics. Some sort of biology experiment, which destabilised to create the original disease (an unknown strain). This disease then mutated into multiple strains such as gravity, light (which split into red, blue, infra red sub-strains), etc. The initial disease would be key to discovering a unified theory in Physics, so is the "unified" strain. The symptoms theoretically generate energy or mass from the infected molecules.