A world where a lot of the landmass is dense jungle like South America. The tribal peoples know about the jungle dangers.Partly they are able to avoid them a lot of the time, partly they are semi-immune to the diseases.Of course every year some die but the majority survive. Frequently they war with each other, over land and water rights, or for other reasons. When they go to war it has different stages. A war might, if it's quick, not go beyond stage one, whilst the longer it lasts the more likely it will reach stage three.Repeated wars turn really nasty much quicker.
In the first stage, fighting is honourable. Captured spies are swapped,ransomed or at worst imprisoned rather then tortured and executed. Magic and biological warfare in the form of dirty weapons are not used-weapons are washed in clean water before battle so wounds are less likely to go septic and rot.
In the second stage, weapons are not washed, spies get worse treatment, civilians are likely to be targeted, towns and villages put under siege,everything gets nastier.
In the third stage, anything goes. Fungus-spears and other poisoned weapons, fire as a weapon, rape,torture and execution, dark magic of the most horrible kind, even on some occasions,genocide of a whole tribe.