He's been dead for years. Inside his body are millions of bacteria that have evolved enough to move around as a colony, and Tom's rotting corpse is just the host. They've replaced most of his organs, except his lungs which they need to take in marijuana smoke, which they feed on. Much like Tom is immune to changing for the better, the bacteria are immune to all medicines. When Tom's body is worn out, we're all fucked, because they'll break out and start multiplying inside other people.
Microbiologists have attempted to study this phenomenon, but none of them could stand the smell long enough to obtain samples.