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As a species, we have an arch nemesis, and we spend a lot of time here on SciShow talking about it: not mosquitoes, though they definitely do suck, or ticks, or asteroids, or zombie apocalypses. I'm talking about viruses. The US is currently experiencing a textbook case of a virus outbreak. It's not particularly big or even severe. Thankfully everyone who has been infected so far has recovered, but it's an object lesson in what makes viruses such a worthy adversary. 

This summer, the US has seen an unusual outbreak of enterovirus, a big, diverse group of viruses that can cause illnesses ranging from summer colds to polio and hand, foot, and mouth disease. It also includes D68, a rare, obscure strain that's causing a rash of hospitalizations, especially among children. D68 is different from other enteroviruses in that it almost exclusively attacks the upper respiratory system. It can cause wheezing, coughing, and flu-like symptoms in young children, but kids with even mild asthma or other immune disorders may quickly become unable to breathe. One of the symptoms being reported is that children's lips turn blue, but what's interesting about this outbreak is how little we know about D68 even though it's not new.

It was first identified in California in 1962, but for the rest of the 20th century, it only occurred in little pockets, and it was basically written off as a really bad summer cold, but it's a hearty little sucker. One trick enteroviruses have that others don't is that they can withstand our stomach acids and even set up shop in our guts. Even though the strain we're talking about right now causes respiratory infections, the "entero" in enterovirus refers to our intestines, but because D68 kept a low profile, it became so obscure that no one kept any records about how many people were being infected or where.

As a result of this low profile, there's not only no vaccine and no widespread natural immunity to it, but the organizations that you'd expect to stay on top of this stuff, like the Centers for Disease Control, turn out to not have any records on the thing, so when outbreaks of D68 were reported in Japan, the Philippines, and the Netherlands in 2009, we were caught off guard, and now, it's back in the US, right at the time when the virus's favorite season, summer, overlaps with the beginning of the school year, providing lots of nose-picking new hosts.

The CDC doesn't know for sure how many people have been infected, but judging by what hospitals are reporting in places like Ohio, Illinois, and Missouri, it's probably in the thousands. The good news is no deaths have been reported in the outbreak, symptoms ease off after a few days, and officials say the outbreak should die down in late October, when the cold weather begins: just in time for flu season!

Now, let me take you back to 2012 and one of SciShow's first episodes ever. It was about overpopulation, and at the time, it was thought that the world's growing population would level off and stabilize by the year 2050, but according to a new study published this week in the journal Science, that's probably not the case. Instead, based on the latest projections from the United Nations, human numbers are expected to keep growing through 2100. By then, we're now expected to have anywhere between 9.6 and 12.3 billion people on Earth. Right now, we've got around 7.2 billion.

So, why has our projected stabilization date extended so far? Well, the answer lies in the statistic known as total fertility rate. That's the average number of children a woman has over her lifetime. The world's total fertility rate has been declining since the mid-1960s due to things like better access to health care, education for women, and revolutions in contraception, but in sub-Saharan Africa, the pace of decline has been less than anticipated, and it's declined less than any other continent. The average woman there has 4.6 children in her lifetime. Compare that to the United States, where the average woman has 2.01 children, or Singapore, where the average woman gives birth to 0.8 children. That's not even a whole child. It's eighty percent of a child.

The authors of the new study attribute Africa's higher fertility rate to a couple of factors. First, for cultural reasons, the notion of the ideal family size tends to be higher in sub-Saharan Africa than in other regions, but also, limited access to education and strong economies to work inside of and limited access to contraception leaves few options for women who don't want to have children, and also, the population is growing because of lower mortality rates, especially from HIV.

As we talked about a few weeks ago, people are dying less from HIV and living longer, due in part to better health care, although southernmost Africa still has the world's highest incidence of the virus, with some countries having more than fifteen percent of the adult population infected, so more people growing older plus new babies being born equals a growing population. Sub-Saharan Africa is now expected to balloon from one billion people now to anywhere between 3.1 and 5.7 billion people by the end of the century.

Thanks for watching SciShow News. We appreciate you watching, but we want you to be even more involved with these episodes, so we're taking a survey of the SciShow audience. You can find the link to it in the description and share with us some random facts about yourself. We promise to reveal the collective results, though with anonymity for all of you, and the odd questions will eventually make sense.