The Chosen live only for a hundred days, but they age according to this rule: their aging is about two hundred and fifty times faster than ours. This is due to a nano-virus Pelops implanted them. This Goa'uld was indeed curious to know how the human body would evolve over the centuries, though he was not patient enough to wait that long.

This is why he selected the Chosen and fastened their aging through the use of a nanotechnology. Each Argosian bears in him millions of nanites, which take from the body what they need to replicate.

During the day, they remain idle and have no effect on their host. However, a transmitter is hidden in the statue inside the temple (see second picture). When the sun goes down, it emits a signal which causes the Chosen to fall deeply asleep while the nanites turn active until morning, when the transmitter sends another signal to turn the nanites off again and wake the Argosians up.

After having intercourse with Kynthia, a Chosen, Jack was infected by the nanites, but because his body was already old (copared to a Chosen which is born already infected), they did not behave the usual way. They replicated more than the usual and could not actually make him older. Therefore they simulated it, causing the effects of aging five to ten years a day. The last picture shows him as he was when the nanites could finally be stopped.

The solution was to use the transmitter. Once it had been discovered, it was easy to definitely turn the nanites off. Then, the immune system would eliminate them, and Jack would become normal again after a few weeks.