The West Nile Virus has affected the bird population in Canada.  At this present time it is not going to cause any major damage but if we let this problem escalate then we may have a larger problem than first anticipated.  Also, we have horses and cows, along with other mammals, including humans, which are becoming infected and ill, or even dying.  Like the birds, this is not causing any problems right now but, like any other small problem, if we let this go on for too long, we will have such a mess we will not be able to control it.