As humans we know asteroids are not the only detrimental events threatening mankind's survival. Disease sweeps the planet on regular intervals inadvertently culling parts of the human population. Science has managed to eradicate many microscopic menaces but still needs to stand on the front line against new strains and mutations. Simply, it's near impossible at this moment in technology to completely rid the world of life threatening diseases and it may not become a reality as we continue to explore distant reaches of the universe.
Discovering new life also warrants the possibility of discovering new diseases. Nature might sometimes attempt to restore balance by introducing variations of diseases capable of abolishing an entire planet of life, and by chance, previously unknown diseases may have hitched a ride with comets and meteors, surviving the rigors of space only to wreak havoc on Earth. Modern theories do suggest life on Earth is the result of water and organisms seeding the planet; Malicious organisms arriving in the same manor isn't too far off.
Maybe there's truly a factor of randomness as with asteroid impacts, that nature once tugged on a giant theoretical machine of creation resulting in a dial of cherry, cherry, orange. Thus, life of a specific type shall appear in the cherry, cherry, orange quadrant of the universe as predefined by a whole slew of other unorthodox methods. In this hypothetical, nature counts how many fractional skips a molecule of helium makes across a partially solidified lava lake measured down to the 4,022nd decimal place. Whatever that outcome is shall be applied as a constant to the machine of creation's results, and may change for the next subset of life to follow as a completely different catalyst is decided. For some reason life exists here of all places in the universe and however that might be determined remains unknown.
† Article 9-41