IDIOT’S GUIDES: SCIENCE MYSTERIES EXPLAINED
Is it true that most of the cells in my body
Since we need various bacteria to help with our digestion, and given that we have all
these mites and other things living in our eyelashes, hair follicles, and creasy bits, are
most of the cells we carry around actually not human?
We do have a lot of hitchhikers, some we need and others that just ride along for the free blood. And yes—
they outnumber us by nearly 10 to 1. And some can be real nasties ….
The scientiﬁcally correct answer to this depends
on how you want to count cells. By weight, you are
deﬁnitely mostly human, around 90 to 95 percent,
depending on whether you’ve had a really good
poop recently (sorry—but that’s nature); there are a
lot of bacteria in feces. But by cell count, only about
10 percent of the cells you haul around everywhere
are your own.
Every person is a mobile ecosystem supporting
a wide range of mites, fungus, bacteria, and viruses
(though viruses don’t have cells). There are not bil-
lions but trillions of nonhuman cells swarming over
your body at every moment. It’s enough to make you
want to take a shower. But don’t, because you might
pick up even more bacteria and fungus from the
Many of these cells live in what’s called a
“commensal” relationship with us. Commensal
literally means “eating at the same table.”
These organisms—mostly mites, fungus, and
bacteria—don’t steal energy from us per se.
They eat the stu we cast o, or even the dirt
that gets on us. Demodex mites, for instance,
live in our eyelashes and eat dead skin. Others
eat our sweat. Some even eat our clothes.
Then there are the organisms that live in a
“symbiotic” relationship with us. This means
they need us to survive, and we need them. The
most important of these are the bacteria in our
gut. We need these bacteria to break down our
food. They get into the dead cells of the plants
and animals we eat and break them open, re-
leasing the chemicals inside. We take the chem-
icals we need to live, and the bacteria gets to eat
the rest. It’s the bacteria’s own metabolism that
produces methane and hydrogen sulﬁde gas in
your intestine, which must be … ahem … passed.
The more you look at the nonhuman material inside
the body, the weirder things get. There are possibly
thousands of species of virus in us that don’t appear to do
anything. But there are also dormant viruses like herpes
that will occasionally ﬂare up and cause a cold sore or
worse. And there are even harmful organisms that can
lie in wait for our immune systems to weaken, and then
pounce. Malaria is a good example—it can go dormant
in the bloodstream and come back months later even if
you’ve left malaria country.
Scientists call this community of dierent nonhuman
organisms in the human body our “microbiome.” And it’s
starting to look like the balance of bugs inside us can have
a massive eect on our health.
We’ve known for some time that taking antibiotics
to knock out a mild respiratory infection can also kill
o huge numbers of our so-called “gut ﬂora.” This is one
reason taking antibiotics can leave you feeling intestinally
More recently, evidence has emerged that the mix of
gut bacteria and the types we have inside us can have a
huge eect on whether we become overweight. It could be
why some people can eat lots of burgers and stay skinny,
while others grow obese.
Creeped out by all those bacteria slithering around
inside you? Think about it this way: you can’t see them or
feel them, and you’d get sick without them. So embrace
your little friends—they might be the best friends you
By cell count,
10% of the
cells in and
on your body