Answer: EVERYTHING!
At the end of HG Well's brilliant novel, the seemingly invincible alien invaders wreaking havoc on humankind are suddenly brought down and overwhelmed by an unseen hand.
This turns out to be the microbes present everywhere, which over aeons humans have learned to live with, but which the aliens have no immunity to.
It's easy to forget that our pets (and ourselves) are covered inside and out with bacteria, fungi, yeasts, viruses and parasites which under the right circumstances can breach the body's natural defences and cause infections that may prove to be serious and even life-threatening.
Given the trillions upon trillions of microorganisms our pets are constantly exposed to - many of whom are waiting for the opportunity to pounce - it's a wonder microbial related diseases don't arise more often than they do. And this is all down to the incredible work carried out around the clock by the ever vigilant immune system, working tirelessly to thwart constant threats.
And the body does this so well, that in most cases there are few or no signs that these battles are even taking place.
Over the last 100 years, the number of potentially harmful 'invaders' dogs and cats are exposed to however, has risen dramatically whether these be microorganisms of one kind or another, or one of the thousands of new industrial substances created every year.
And it's for this reason we need to proactively support our pet's immune system, if it is to continue to protect the body as effectively as possible against microbial / chemical attacks from outside and within, plus the 'alien' cells of tumours and cancers.
Immunity and Detoxification Support are powerful allies in this fight.