Planet of Viruses, Third Edition

This three-way hybrid continued to circulate among Mexican pigs for years.
Scientists estimate that it finally jumped into humans in the fall of 2008

And when two different flu viruses reproduce inside the same cell, things can get messy.

The genes of a flu virus are stored on eight separate segments.
When a host cell starts manufacturing the segments from two different viruses at once, they sometimes get mixed together.

Most of these invasions end in failure, though.
The genes a bird flu virus needs to thrive are different from those needed inside a human body.
Human bodies are cooler than bird bodies, for example, and that difference means that molecules need different shapes to run efficiently.
As a result, bird flu viruses may replicate slowly in our bodies, making them easy prey for our immune systems.

The subtype that emerged back then, known as H1N1, made its way from humans to pigs, and it continued infecting pigs long after the human pandemic ended.
As pigs were shipped from country to country through international trade, the H1N1 subtype spread into new herds, mutating along the way.

In the 1990s, pigs from both Europe and North America were important to Mexico.
Each stock of animals carried its own version of H1N1.
And in Mexico, those two kinds of influenza mixed their genes through reassortment in pigs.

he flu vaccine can dramatically lower the odds of such a tragic outcome.
It is made of the proteins that stud the surface of influenza viruses, prompting the immune system to prepare antibodies before we get infected with real viruses.

Human rhinoviruses may help train our immune systems not to overreact to minor triggers, instead directing their assaults to real threats.

But should they?
The answer is actually not clear.
Human rhinoviruses impose a serious burden on public health, not just by causing colds, but by opening the way for more harmful pathogens.
Yet the effects of human rhinovirus itself are relatively mild.

Parents often give children cough syrup for colds, but studies show it doesn’t make people get better faster.
In fact, cough syrup poses a wide variety of rare yet serious side effects, such as convulsions, rapid heart rate, and even death.

We have only ourselves to blame.
Infected cells release signaling molecules, called cytokines, which attract nearby immune cells.
Those immune cells then make us feel awful.
They create inflammation that triggers a scratchy feeling in the throat and leads to the production of mucus around the site of the infection.

rhinoviruses can latch on to the cells that line the nasal passage.