Headscratchers / Non-Stop

• Why don't the Intruders suffer any ill effects of the 9-day fever which caused the mutation of ships original crew. Intruders have to maintain their cover so they have to eat the same stuff the inhabitants do. But the ship's ecosystem is adapted to the alien protein. Shouldn't all Intruders get sick too?
• Considering the enormous havoc which the mutated intelligent rats wreak aboard the ship, makes one wonder, why did the people take rats and other vermin aboard and fare them all the way to Procyon in the first place?