The birds are singing.

It looks like the start of another lovely day.

You're walking happily in the park, when, "Ah-choo!"

A passing stranger has expelled mucus and saliva from their mouth and nose.

You can feel the droplets of moisture land on your skin,

but what you can't feel are the thousands, or even millions, of microscopic germs

that have covertly traveled through the air and onto your clothing, hands and face.

As gross as this scenario sounds,

it's actually very common for our bodies to be exposed to disease-causing germs,

and most of the time, it's not nearly as obvious.

Germs are found on almost every surface we come into contact with.

When we talk about germs,

we're actually referring to many different kinds of microscopic organisms,

including bacteria, fungi, protozoa and viruses.

But what our germs all have in common is the ability to interact with our bodies and change how we feel and function.

Scientists who study infectious diseases have wondered for decades why it is that some of these germs are relatively harmless,

while others cause devastating effects and can sometimes be fatal.

We still haven't solved the entire puzzle,

but what we do know is that the harmfulness, or virulence, of a germ is a result of evolution.

How can it be that the same evolutionary process can produce germs that cause very different levels of harm?

The answer starts to become clear if we think about a germ's mode of transmission,

which is the strategy it uses to get from one host to the next.

A common mode of transmission occurs through the air, like the sneeze you just witnessed,

and one germ that uses this method is the rhinovirus,

which replicates in our upper airways, and is responsible for up to half of all common colds.

Now, imagine that after the sneeze,

one of three hypothetical varieties of rhinovirus,

let's call them "too much," "too little," and "just right,"

has been lucky enough to land on you.

These viruses are hardwired to replicate,

but because of genetic differences, they will do so at different rates.

"Too much" multiplies very often, making it very successful in the short run.

However, this success comes at a cost to you, the host.

A quickly replicating virus can cause more damage to your body,

making cold symptoms more severe.

If you're too sick to leave your home,

you don't give the virus any opportunities to jump to a new host.

And if the disease should kill you, the virus' own life cycle will end along with yours.

"Too little," on the other hand, multiplies rarely and causes you little harm in the process.

Although this leaves you healthy enough to interact with other potential hosts,

the lack of symptoms means you may not sneeze at all,

or if you do, there may be too few viruses in your mucus to infect anyone else.

Meanwhile, "just right" has been replicating quickly enough to ensure that you're carrying sufficient amounts of the virus to spread,

but not so often that you're too sick to get out of bed.

And in the end, it's the one that will be most successful at transmitting itself to new hosts and giving rise to the next generation.

This describes what scientists call trade-off hypothesis.

First developed in the early 1980s,

it predicts that germs will evolve to maximize their overall success by achieving a balance between replicating within a host,

which causes virulence, and transmission to a new host.

In the case of the rhinovirus,

the hypothesis predicts that its evolution will favor less virulent forms

because it relies on close contact to get to its next victim.

For the rhinovirus, a mobile host is a good host,

and indeed, that is what we see.

While most people experience a runny nose, coughing and sneezing,

the common cold is generally mild and only lasts about a week.

It would be great if the story ended there,

but germs use many other modes of transmission.

For example, the malaria parasite, plasmodium, is transmitted by mosquitoes.

Unlike the rhinovirus, it doesn't need us to be up and about,

and may even benefit from harming us since a sick and immobile person is easier for mosquitoes to bite.

We would expect germs that depend less on host mobility,

like those transmitted by insects, water or food,

to cause more severe symptoms.

So, what can we do to reduce the harmfulness of infectious diseases?

Evolutionary biologist Dr. Paul Ewald has suggested that we can actually direct their evolution through simple disease-control methods.

By mosquito-proofing houses, establishing clean water systems,

or staying home when we get a cold,

we can obstruct the transmission strategies of harmful germs while creating a greater dependence on host mobility.

So, while traditional methods of trying to eradicate germs may only breed stronger ones in the long run,

this innovative approach of encouraging them to evolve milder forms could be a win-win situation.

Well, for the most part.