Sentence:  Every mammal on this planet instinctively develops a natural <WSD>equilibrium</WSD> with the surrounding environment but you humans do not. You move to an area and you multiply and multiply until every natural resource is consumed and the only way you can survive is to spread to another area. There is another organism on this planet that follows the same pattern. Do you know what it is? A virus. 
Meaning of the word marked with <WSD> tags:  The condition of a system in which competing influences are balanced, resulting in no net change.