Every CE symptom needs a START time. An END time is optional, allowing you to impart permanent buffs and/or disabilities if you don't include one.
[CE_CAN_DO_INTERACTION:START:0]
PEAK only makes sense for symptoms with a SEV value - there's no severity for binary effects like transformation.
[CE_PAIN:SEV:75:PROB:100:RESISTABLE:SIZE_DILUTES:LOCALIZED:VASCULAR_ONLY:START:10:PEAK:50:END:1200]
While all PEAK uses I know of are fit between a START and END time, theoretically an endless symptom with a PEAK time would slowly ramp up to the full extent of the curse/deformity. Testing is required there.
If you delay the start time, you can have some interesting layered effects. For example, here's a "rabies" bite I added for ice wolves, which applies only to dogs; it applies a fever for 500 ticks, and at the end of the fever it has a 1/3 chance for the creature to permanently turn Crazed, requiring an Old Yeller-type solution either before or right after the end of the fever.
[CE_FEVER:SEV:100:PROB:100:RESISTABLE:SIZE_DILUTES:START:5:PEAK:400:END:500:DWF_STRETCH:4]
[CE_ADD_TAG:CRAZED:PROB:33:RESISTABLE:START:499:DWF_STRETCH:4]
A similar effect; I made it so eating a dragon's heart gives you fire breath. However, you must first endure a fever, which prevents you from just munching it mid-combat. A similar approach could be used to make other abilities more interesting to acquire, such as rotting off body parts/tissues off before transforming into a new creature.
[CE_FEVER:PROB:100:SEV:100:START:0:END:25:ABRUPT_START:ABRUPT_END:DWF_STRETCH:4]
[CE_DISPLAY_NAME:NAME:dragonblood:dragonbloods:dragonblood:START:25:DWF_STRETCH:4]
[CE_CAN_DO_INTERACTION:START:25:ABRUPT_START:DWF_STRETCH:4]
[CDI:INTERACTION:MATERIAL_EMISSION]
[CDI:ADV_NAME:Breathe fire] ~etc.