The von Neuman seeding probe was a genetically engineered probe to either terraform other planets or be used as a weapon. It was named after John von Neumann, a scientist from Earth who theorised about such a creation.
The probe worked in three stages. The first was biomass assimilation and consolidation, breaking down organic matter. The more organic matter it could break down, the more complex its internal structure could become. The second stage was creating simple constructs using the data it had assimilated. The third was the "enquiry stage", a fail safe to make sure the probe couldn't knock out an intelligent species. The probe asked a question — explain ratio 1:812 — and if the question was answered correctly then the probe would self-terminate.
More than 1000 years before the Doctor was born, the probe landed on Gallifrey and killed several hundred thousand Time Lords. The Time Lords eventually contained the probe, and, to make sure such a catastrophe never happened again, they engineered an immunity into their genes.
Not long after 1947 — during the age of UNIT and the internet — the probe landed on Earth at a place called Fort Casey. It wiped the whole town out, killing every organic thing inside it, as well as three of Major Platt's investigation team. The Eighth Doctor, making use of his immunity, successfully answered the failsafe question and the probe died. (PROSE: Spore)