People seem to forget about the quarantined planet. Also the fact that 6 has no such healing powers.
On the planet under quarantime they were conducting medical experiments. They were trying to create a "cure all" drug. Part antivirus/antibacteria, part mutagen, it was supposed to cure "everything" making disease history. After the disaster wiped out the scientists, the GA put the planet under quarantine.
When the crew of the freighter went to retrieve the research data, somebody got infected. During the ensuing battle, most of the ship is destroyed, releasing lethal levels of radiation. This exposure causes the virus to mutate.
When 2 gets infected, it's by the newly mutated virus. It's effect is profoundly different. Not only can it rewrited a hosts' DNA in less than 3 hours, it can mask itself in their genetic makeup making it undetectable.
