Jack: "I'm not sure what's going on. But everyone seems to be complaining about not feeling too good. I suspect its some form of nasty flu. But I think I'll have Brenda take some blood, and check the water supply just to be on the safe side."
Brenda: "Our worst suspicions have been confirmed: Lucas has been poisoning the water supply. My father asks that you secure it from its source immediately."
Signs that something is wrong begin with complaints of a flu. This manifests in a subtle drop in performance in your mercs and in any guards you have placed. While in this condition, every action they take costs extra breath and this breath takes longer to recover. Although the poison's effect is not huge, it is noticeable as it adds up. The severity of this breath penalty varies by difficulty level.
In time, Brenda informs you that the water supplying the base compound has been tainted, and recommends you recapture the water supply in the north so that the water can be cleansed and everyone's condition restored.
This mission is fairly straightforward - capture all five Reservoir sectors (Sectors 29, 30, 39, 49 and 50), and the quest will be completed.
Jack: "Securing the fresh water supply was a step in the right direction. Let's hope it doesn't take too long for us to feel the benefit!" Jack: "I can't tell you how good it feels to be finally rid of that...condition! The men should now be able to pay full attention to the job. I think its extremely important that we don't allow Santino the opportunity to pull that stunt again!"
Jack will congratulate you the day you recapture the water source, and two full days after, will report that the effects of the sickness have passed. This results in an overall increase in performance of your mercs and guards.
Should you let the enemy recaptures any one of the five Reservoir sectors, your mercs and guards will fall prey to the illness once more until the Reservoir sectors are secured again.