If STD's are the best that nature can come up with, I'd say it's doing a pathetic job of killing us off. The human population of the world is growing at an incredible rate.
Think of the death toll in Iraq - let's make it totally horrendously overblown, at 500,000 people. Based on a current world population of about 6.5 billion, that's 0.0077%. Conventional warfare isn't particularly efficient either.
Let's say terrorists get ahold of a bunch of hydrogen bombs and wipe the US off the map completely. ~300M people gone. That's about 4.6% of all humans gone. It's a start.
Even in the event of all-out nuclear warfare, some humans would likely survive somewhere. The population could drop significantly, but humans will probably live somewhere, unless we opt to exhaust
all of our nuclear weapons and irradiate the entire planet very thoroughly.
Think about it. If you take away the ability to procreate, reproduce and multiply...
Well sure, if you do all that, then yeah, eventually the living generation will age and die. That'll hold true for any life form. But STDs are far from that. Hell, look at Africa. HIV is thriving there, but people are still having babies.
Thus far, I'd say that nature sucks at killing humans. Until nature sends a large asteroid our way, which our under(un?)funded NEO search program wouldn't see in time to do anything about it. That is really the only threat I see to our species in terms of extinction.