The virus as described by Dr. Phlox would change their DNA, but changing DNA does not result in immediate or complete anatomical change. They would likely have no immediate physical symptoms. If the illness was not treated, they would die in the incomplete transformation process, failing the purpose of the engineered virus.
During a neuropressure session, Trip complains that T'pol's hands are "ice cold", so she rubs her palms together to warm them. However, her next pressure move is applied with the (unwarmed) back of her hand, yet Trip doesn't react.
At 31:16 as the alien formerly known as Archer walks through the crevice there is a crew member's side of body & arm/hand softly motioning for the actors to stay in place. Like to be Woman wearing a black overcoat.
When Trip tries to persuade T'Pol to come back to Enterprise with him, she refuses, saying she'd rather stay with Archer and Hoshi, but T'Pol is completely out of contact with Enterprise while she follows her two crewmates. Why doesn't she take a communicator and phaser from Trip so that she's at least able to communicate with Enterprise and defend herself?
Enterprise can detect the bio-signs of the altered Captain, Reed and Hoshi from orbit, but they provide no guidance or information to the team that lands with shuttle-pod 2.
After T'Pol witnesses Reed, Archer and herself being transformed she immediately orders a security team down to the planet, which is extremely illogical given it would expose them to whatever is transforming the away team as well.