Humans are selfish and ultimately rather stupid animals, just like the so-called "lower" species. Currently, they are just much more destructive than other species: so I'm not sure if 'disease' is the right word, unless you want to describe all life as a disease. (Although in a different context you could say that life is a sexually transmitted disease...) The only ultimate difference is that humans (eventually) will be able to colonize other worlds, barring global catastrophe. Whether that's a good thing or a bad thing is not clear.

I should say, though, that birth control is slowing population growth fast: the growth rate has already nearly ceased in Europe, and would be here to if it weren't for Latin American immigration. Once the third-world countries modernize their societies, population growth will cease there too. I think the UN estimate for the peak population, which will occur around the start of the 22nd century, is somewhere around 10-20 billion. Way too damn many, of course, but it could be much worse. And even if not for that, the Malthusian crunch can't be averted forever.

The funny thing is that humanity will never end, in essance. This is because when there isn't a human left, there won't be anyone to say "humanity is gone"... it would almost be as if it never existed. Humanity exists because we are here saying that it does. Words are arbitrary, yes, but this is even deeper than that. If humanity ceases to exist, there will be no one to know that it doesn't exist anymore, therefor it never did exist. IT's all a matter of perception.
You don't think other animals are sentient, or even that they could become so with the right evolutionary pressure?