Cable's Relationship To The Techno-Organic Virus
What would be the reason behind infecting Nathan with the virus? To kill him? More likely not! I would see that Apocalypse was doing was to test the child's worth. If the child survives, surely there can be of some use for the ancient warlord right? Nathan possessed so much power that he might prove his worth in the midst of that infection. When he was taken into 2,000 years into the future by his alternate reality half-sister Rachel, he miraculously survived the infection which didn't hit until he hit puberty. The fact that Nathan could survive the infection and that his powers could hold the T-O Virus at bay proves that he was indeed, a very powerful telepath. He would have to use his powers to hold the virus at bay.
Because of the virus, Cable could not fully use his power which may have lessened Apocalypse' chances of being overthrown. Cable would also be pretty outmatched by his more powerful clone called Stryfe. Stryfe unlike Cable could fully manifest the potential of the Summers-Grey DNA while Cable could not due to the techno-organic virus. Both enemies still clashed and at times, Cable used limited use of his telepathy and telekinesis because of the virus. During Psylocke's battle with the Shadow King, Cable almost died as all telepathic powers were blocked for a time. Later he would the Psimitar spear to harness his potential... and he would later use that weapon to dissipate Apocalypse' consciousness. But later, Cable would use the T-O Virus to reawaken his ancient foe after the House of M incident.