In War for the Planet of the Apes, the Colonel is trying to avoid the spread of the disease which turns talking humans to non-talking beasts. He claims his son went crazy, turned into a range beast, and he had to kill him. The plot revolves around this strange virus mutation and its effects.
However, we see at least three cases (2 short, 1 long) that the disease just removed the humans' ability to speak, not their actual intelligence or rational behavior.
The kid, Nova. Her dad ran from the soldier group months ago, but is still able to speak (talking about some sticks, "I'm just going to put these down"). The girl seems completely normal, aside from not speaking. She is smart, funny, she saves the day, she learns sign language
The soldier killed in the snow. When Caeser and crew find 3 corpses, one is still alive. He looks at them, scared. He then relaxes when he understands the apes are going to end his suffering.
The Colonel himself. Not being able to speak, he ends his own life
Doesn't seem that much of a beast behavior to me. Granted, the soldiers and Colonel might have been affected for a very short time, but Nova at least, doesn't exhibit beast-life behavior. So, how bad is this disease, really?