For example, if I say "I have proven that vitamin Q prevents heart disease," that is a falsifiable claim. It would be relatively easy to prove that the mythical vitamin Q does NOT prevent heart disease. This makes my claim legitimate, assuming that I can make my case of course.
However, if I say "Vitamin Q was planted on Earth by space aliens two million years ago," that is NOT a falsifiable claim. There is no way to prove that vitamin Q was NOT planted here by space aliens. Since this claim cannot be proven false, it has no scientific legitimacy.
We tend to think of mosquito borne diseases as "someone else's problem." Malaria kills two million children under five every year, but most of the deaths are in the developing world. West Nile Virus had a lot of hype in those early years, but most of the hype has settled down and we didn't all die from it, so how big a deal could it be?
Mosquitos are a remarkably efficient transmitter of diseases. They suck the blood from one warm-blooded creature, and then dribble some of it back into their next victim. It's like you're getting backwash, but it's the blood of a complete stranger.