Speak to Alchemist Pestlezugg in Gadgetzan.
It would seem that the silithid's intent, nay - reason for being, is to devour everything in their path that is not of their ecology. Their focus is singular, and it is for this reason they are a grave threat to all life on Azeroth.
What we've seen to date is but the first stages. I suspect that their strength comes from deeper in Kalimdor: Un'Goro Crater. Go to Gadgetzan in the Tanaris desert and speak to Alchemist Pestlezugg. Tell him of my theory; he will surely aid us in fighting the silithid.