You could have it where, if the test to diagnose is not a superior success, then Stage 2 goes unnoticed. And if the test fails as a Complete Failure, stage 2 is triggered and comes on more rapidly.
Here's an attempt at game-stats. Feel free to toss or tweak...
Tal Shiar "Zombie Virus" (I'm sure they have another name for it in Romulan). Invented by Romulan geneticists in the mid-2350s, and perfected by testing on Reman subjects, as well as captured Klingons.
The Virus is spread by being ingested through food or drink and can only survive in certain substances (explaining why only certain members of the crew are effected - the lead PCs, by sheer luck, didn't replicate or ingest anything the virus could survive in); and delivered by a device installed by a Tal Shiar agent.
Onset: 2d6+Vitality mod / hours
Potency: +7 TN
Diagnosis: +7 TN
Treatment: +9 TN
Effects:
Stage 1 - the victim becomes "zombie like", and suffer a -5 to all Intellect attribute tests, and all tests with Intellect-based skills. The same penalty is imposed on Perception tests, and skills based on that attribute. At this stage the virus survives in the host by feeding off of oxygen in the blood, causing a cyanotic condition in the host, making them have a very "undead" appearance.
Stage 2 - The virus begins to take its toll, reducing the victim's Wound Levels at a rate of 2d6 per day. At this point if the victim's Wound Levels drop to 0 he will slip into a coma, and die.
That's just off the top of my head and very heavily influenced by DS9's Aphasia Virus, which is detailed in the Narrator's Guide (page 224).
Use any, all, or none as it suits your adventure