The world is full of undiscovered viruses. They’re in the air we breathe, the ground we walk on, and they’re inside our bellies. For this last episode of the season, we’re exploring the mysteries of the microbes that have us surrounded. First we meet Portland State University virologist Ken Stedman, who made a wild discovery that changed what we thought a virus could be. Then, Shiraz Shah from the Copenhagen University Hospital Gentofte explains how viruses that colonize our guts during infancy may affect our health for the rest of our lives.

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We thought we knew everything there was to know about measles. But in recent years, new research has revealed that the virus attacks the immune system and creates effects far more dramatic than a rash and fever. For this episode we’re joined by Michael Mina, a former Harvard epidemiologist now at eMed, who helped discover how measles was causing “immune amnesia." Our second guest is Stephen Russell, a former Mayo Clinic researcher who co-founded a company called Vyriad. Russell is trying to use the measles virus to treat cancer.

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For thousands of years, humans have shaped mosquito evolution while mosquitoes have shaped human history. Today on the show, Noah Rose, an ecologist at UC San Diego, tells us how mosquitoes came to love human blood. Then, Georgetown historian John McNeill makes the case for how mosquitoes – and the viruses they carry – changed the course of history in the Americas.

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After four decades of dedicated research on HIV, scientists have made extraordinary progress in treating the disease. But we still don’t have a vaccine or a cure. On today’s show, we’re joined by two veteran scientists who have dedicated their careers to HIV research. First up is Christine Rouzioux, a virologist from the Nobel Prize winning team of scientists who first identified the HIV virus. For the second half of the show we talk with Richard Koup from the National Institutes of Health, who explains why it’s so hard to create an HIV vaccine.

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For a long time, people could tell that there was some connection between chickenpox and shingles. But exactly how they were related was a mystery. Then, in the 1950s, a family doctor shipped out to a remote Scottish island to investigate an outbreak, and made a discovery that shaped our understanding of shingles. On today's show, Ann Arvin, professor emerita at Stanford Medical School, tells us that detective story. Then Robert Johnson of the University of Bristol explains what he's learned about treating pain in his decades working with shingles patients.

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Almost everyone on earth has Epstein-Barr virus. Usually it's pretty mild - you get sick, you get better. But the virus lives on in your body forever. Today, we talk with Dorothy Crawford (author of "Cancer Virus") about how one obsessive researcher uncovered a link between Epstein-Barr and cancer, and changed the way we think about viruses. Later in the show, we talk with Bill Robinson about his groundbreaking discovery of how Epstein-Barr can cause multiple sclerosis and other autoimmune diseases.

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What did the 2014 Ebola outbreak teach us about preventing future pandemics? Our guests this week, Christian Happi and Pardis Sabeti, are world experts on disease surveillance, and have worked together fighting infectious disease in Africa for over a decade. Happi shares a gripping account of how he courageously helped stop Ebola from spreading in Nigeria during the 2014 West Africa outbreak. Then Sabeti explains how a new era of surveillance may help prevent future pandemics.

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Why has rabies invaded our nightmares for centuries? Author and veterinarian Monica Murphy tells us about the cultural history of rabies (which involves vampires and werewolves!) and how our long nightmare with the disease came to an end. Then, wildlife biologist Kathy Nelson tells us about a surprising program that works to control raccoon rabies… from the sky.

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