cub_craft
of reading
42
Read books
i remember seeing a reddit post about this a while back and one of the replies was a guy thinking that Dr. Gero engineered it so Goku was weakened and he could take over the world with the androids or something similar. But i think that it was simply a regular virus on earth due to them having a vaccine about 20 years later so depends on what u think is more plausible/fits with the plot best.