Pathogens are everywhere

We think that pathogens – viruses, bacteria, fungi, and protozoa – are, along with toxic and malnourishing diets, the main cause of human disease.

People who think we exaggerate the impact of microbes on health may not have fully appreciated the ubiquity of these pathogens. We live in a sea of microbes, many of whom would like nothing better than to live at our expense.

So today, let’s look at just how abundant microbes are.

In the Water

When you swim in the ocean, how many viruses are you swallowing?

… pause … time for reader to guess …

The answer is in a fascinating story in The Scientist:

Once thought not to exist in marine environments, scientists now realize that there are some 50 million viruses in every milliliter of seawater.

These viruses can not only infect cellular life, they frequently kill it:

Every day, marine viruses kill about 20 percent of the ocean’s microorganisms, which produce about half the oxygen on the planet.

It’s not just viruses: Vibrio cholerae, the bacterium that causes epidemic cholera, is widespread in ocean water, and is the most common cause of food poisoning from eating shellfish.

In the Air

What about the air?  How many microbes do you inhale when you breath?

… pause … time for reader to guess …

The Scientist once again came to my aid:

Every cubic meter of air holds upwards of 100 million microorganisms …

Lungs contain about 2.4 liters of air, of which 0.5 liters is expelled every breath. A cubic meter has 1,000 liters, so a single breath takes in 50,000 microorganisms.

Some more information for the curious:

Recent research published in PNAS suggests that the diversity of microbial life in the air is on par with the soil, at least in urban areas, yet the air remains vastly understudied in comparison.

“Just seven or ten years ago we didn’t realize bacteria existed in clouds,” said Anne-Marie Delort, professor of microbiology and organic chemistry at Université Blaise Pascal in France. Now researchers know microbes act as a surface for the condensation of water vapor in the atmosphere, thus forming clouds. Recent research publish in Science shows microbes also play the same role during snowflake formation and other types of precipitation.

Which Is More Dangerous, Air or Water?

Not all microbes flourish in the human body, but all have to be dealt with by our immune defenses. And some can, and do, establish lasting infections in humans.

Since both air and water have pathogenic microbes, it seems fair to ask which environment is more likely to make you sick.

Luckily scientists have done a controlled trial. [1] They sent two sets of people to the beach, and instructed half to remain in the air and the other half to venture into the water. ScienceDaily has details:

A yearlong beach study led by a team of University of Miami researchers suggests that swimmers at sub-tropical beaches face an increased risk of illness….

B.E.A.C.H.E.S. (Beach Environmental Assessment and Characterization Human Exposure Study) enlisted more than 1,300 volunteers, all local residents who regularly use South Florida beaches. Researchers divided study participants into two groups: volunteers who went into the water and those instructed to stay out of the water. The group that went in the water was asked to dunk themselves completely in the water three times over a fifteen-minute period. A few days later both sets of participants received follow-up calls from researchers, checking on their health and well being.

“We found that when swimming in sub-tropical beach areas with no known pollution or contamination from sewage or runoff, you still have a chance of being exposed to the kind of microbes that can make you sick,” said Dr. Lora Fleming …

The study found that the swimmers were 1.76 times more likely to report a gastrointestinal illness, and 4.46 times more likely to report having a fever or respiratory illness. Swimmers in the study were also nearly six times more likely to report a skin illness than those volunteers who stayed out of the water.

The obvious flaw in this study was the lack of a control group placed in a vacuum. It would have been nice to know if complete isolation from microbes would have improved health even further. Perhaps the scientists lacked funding for this third group.

(Warning: inside joke coming.) Of course, it may be impossible for this study ever to be replicated in the US, since after these results how can an ocean swimming group ever be permitted by an Institutional Review Board? It seems that follow-up studies will have to be performed on foreign beaches, perhaps in Rio, the French Riviera, or Tahiti.


It seems the microbes have us surrounded. Whether you venture into the air or the water, you have a chance to get sick.

Is there anything you can do to protect yourself, besides staying home and cowering under your bed? Possibly. We’ll look into that in upcoming posts.


[1] Fleisher JM et al. The BEACHES Study: health effects and exposures from non-point source microbial contaminants in subtropical recreational marine waters. Int J Epidemiol. 2010 Oct;39(5):1291-8.

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