How can I decrease Severity?

Solution 1:

You can maintain low severity by devolving any symptoms you mutate. The only other way to reduce severity if you are a Bio-Weapon or a Parasite as they both have traits that can be evolved to reduce severity.

EDIT: Forgot about Neurax Worm being able to reduce severity (Stolen from Rokk's answer)

Solution 2:

For the Neurax worm, certain symptoms actually decrease severity. Most notably, the first few symptoms down the lower left path (such as Occipital Mesh) decrease severity, unless indicated otherwise.

If you are going for a stealthy approach with the Neurax worm, make sure you evolve these symptoms. And keep transmissions to a minimum, as the Neurax Worm's transmissions increase your severity.