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Hank: Viruses are bags of genetic material that are caught between life and non-life. They need living cells to replicate and spread. Lots of them make us sick, like the flu, measles, or HIV. And, as a species, we’re getting better at preventing viral diseases.

But eradication – or, eliminating them completely – is much harder. We have done it before, though! The eradication of the deadly smallpox virus – declared by the World Health Organization in 1980 – is hailed as one of humanity’s greatest achievements. There’s just one other time humans kicked viral butt: rinderpest, or cattle plague.

Vaccines and global efforts beat this lethal virus for good, and we learned a lot along the way. But every virus presents a different challenge, so each one needs lots of research to come up with effective treatments.

Rinderpest was a nasty livestock disease. Infected cattle had a high chance of dying, suffering from a mix of fever, diarrhea, and discharge from almost every orifice. All those bodily fluids helped the virus spread... and it really spread. Originally from Asia, rinderpest slowly moved into Europe, and became an epidemic in Africa in the 1880s. In Ethiopia, rinderpest wiped out a third of the human population, by killing the animals they depended on for food and transport.

And it didn’t go away. Rinderpest caused devastation across these three continents, and fear everywhere else. Quarantine and basic hygiene strategies helped a little, but what we needed was a vaccine – a little dose of weakened virus, to train cattle immune systems to recognize and attack the enemy.

An early vaccine from the 1920s used a technique called serial passage, making a weakened form of rinderpest by growing the virus in multiple animals or cells in a lab. In this case, the scientist J.T. Edwards infected a different species, so the virus evolved to suit goats rather than cattle. If you then put this modified virus back into a cow, the cow’s immune system can safely fight it, and build up anti-rinderpest antibodies in case the normal virus comes along.

But the method takes a long time and meant researchers had to look after a whole herd of goats, which isn’t practical for mass vaccination. Plus, sometimes the virus would partially revert back to its original form, and still cause disease.

By the 1950s, we knew how to grow cells in petri dishes – a technique known as tissue culture. Using time, luck, and serial passage in lab-grown calf kidney cells, vet scientist Walter Plowright developed a new rinderpest vaccine, riddled with a bunch of weakening mutations. And! Best part! No goats necessary! It worked more reliably, and rinderpest infection rates plummeted in the next few decades.

But without a cooperative global effort, bouts of rinderpest kept popping back up, like a deadly whack-a-mole for cows. So in 1994, the Global Rinderpest Eradication Programme was launched. Its mission: to target rinderpest’s major outposts, then destroy it completely. The program’s success depended on science and society, at local and international levels.

Researchers studied how the virus worked and spread, and used mathematical models for more strategic vaccination, rather than trying to vaccinate as many cattle as possible. Animals were injected with a new heat-stable form of the vaccine, which didn’t need to stay refrigerated. Outside the lab, local farmers were really involved in educating people about rinderpest. They made up a surveillance network, to look out for resurgences of the disease or confirm healthy areas. And in 2011, 10 years after the last recorded case, rinderpest was officially declared eradicated... at least in the wild, since some lab samples still exist.

We learned a lot during our long but victorious battle against rinderpest. So why haven’t we eliminated all viruses? Well, even though it was devastating, rinderpest was actually more of an “entry level” viral problem: Rinderpest mutated relatively slowly, and didn’t vary much between strains. So one-vaccine-fit-all, and immunity lasted a lifetime. Take influenza viruses, for comparison, which change their genetic material so much that new vaccines are made each year to keep up.

Also, rinderpest’s horrible symptoms were obvious, making it pretty easy to find outbreaks and isolate infected herds. Meanwhile, viruses like Zika can spread quietly through asymptomatic carriers, and other viruses like HIV lie low for years before taking their toll. These viruses are much harder to track.

So, rinderpest may be a thing of the past, but the world is still full of viruses, and the future is still unknown. As we’ve seen from HIV, Zika, and Ebola, new viruses can seemingly pop up out of nowhere and spread fast. When they do, the race is on to understand the viruses’ activity, strengths, and weaknesses – so we can find the best strategies to prevent infection and disease.

We’re getting better at it, thanks to better technology and communication networks. But each virus still presents unique threats and challenges. With food, welfare, and lives at stake, we need to remember that science and global policy work hand in hand to stamp out deadly viruses for good.

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