This page of the guide for Resident Evil 3 Remake has information on how to get rid of the parasite that can infect Jill. The infected heroine will be much weaker. Getting the parasites out of her body as quickly as possible is crucial to keeping her alive.
Jill may become infected with the parasite by a Drain Deimos. By default, the only location where these enemies appear is the Subway Power Substation in Raccoon City. This location is visited during the main storyline - you have to solve the puzzle with 4 generators.
The first infection occurs when you enter the area with generators - after opening the gate with the lockpick. Jill, however, may be infected again even after you can cure her. You just have to avoid situations where a spider-like creature can jump on her. You can defend yourself in two ways - by killing Drain Deimos or jumping sideways when they start jumping.
The infected Jill moves slowly and this condition is further symbolized by the red tint and veins on the edges of the screen. This makes the heroine an easy target for the creatures.
To get rid of the parasite, you need to use a green herb or other medicine. This removes the parasite and helps the heroine move normal again. You can find plants in substation 2 even before you open the gate. Another green herb is in the area with generators - pick it up if you run out of all the medicines.