Oh they all try to change the past. And it would make no difference if they didn’t try to change it. It’s just that their efforts to change the past are already included in the past. The past and the future are quite bound together. A future person going back can “do” things, seemingly making decisions along the way, but their influence must already be part of the future’s history. All the time travellers can influence is by way of memories brought back. The scientist cannot stop the virus, but she can discover where a pure sample might be found (or discover that she already knows, but did not realise the significance of her memories). She might try to stop it, but that would just contribute to the way it already was/ would be released.
The time travel does nothing more than uncover the significance of memories and records. This was particularly nicely done in the scene where Stowe makes the ‘phone call that set them onto the track of the Army of the 12 Monkeys in the first place. Willis’ interaction with her - which required him having been sent back - led to this call, which led to him being sent back. Neat and consistent (but really bad news for free will). The only slightly clunky bit is the gradual decoding of new material.