# What is a vaccine?

An episode of [Science for Kids / Science's Not Boring: The Kids Science Podcast](https://invisiblenetwork.ai/podcast/1896928296), 4 Aug 2026, 11 min.

Audio: https://www.buzzsprout.com/2624612/episodes/19562057-what-is-a-vaccine.mp3

Have you ever wondered what happens inside your body when you get a scrape or catch a cold? Get ready to shrink down to microscopic size, because there is an epic battle happening inside you right now! Your body has a built-in security team with billions of invisible hunters, including giant white blood cells called macrophages that eat germs like hungry bears! But how does your body learn to fight off the really tough bad guys? We are traveling back in time to 1796 to meet Dr. Edward Jenner, a milkmaid named Sarah, and a very special cow named Blossom. Discover the incredible true story of how a mild sickness called cowpox helped defeat a terrifying disease called smallpox. You will find out how scientists discovered they could give your immune system a safe practice run by handing your cells a "wanted poster" of the germs, creating the world's very first vaccine! From giant germ-eating cells to memory cells that patrol your blood forever, join Mira and Finn for a fascinating journey into the microscopic superhero movie happening inside you every single day!

## Transcript

[0:00] SPEAKER_00: Welcome to Science Is Not Boring by Kidopoly.com.
[0:03] SPEAKER_00: I'm Mira! And I'm Finn! Finn, look at the tip of your finger.
[0:09] SPEAKER_00: Imagine a single drop of blood right there, no bigger than the head of a pin.
[0:14] SPEAKER_01: Okay, I'm imagining it.
[0:16] SPEAKER_01: It's just a tiny red dot.
[0:19] SPEAKER_00: Well, inside that one tiny drop, you have up to 10,000 invisible hunters constantly patrolling your body.
[0:27] SPEAKER_00: And if we count your whole body, you have billions of them.
[0:32] SPEAKER_01: Billions?
[0:33] SPEAKER_01: That's more than the number of people on Earth.
[0:35] SPEAKER_01: What are they hunting?
[0:37] SPEAKER_00: Germs! Today we are talking about why we get sick, how your body launches a massive defence you don't even notice, and how a scientist, a milkmaid, and a cow named Blossom change the course of human history forever.
[0:53] SPEAKER_00: Wait, a cow?
[0:54] SPEAKER_00: Did the cow invent medicine?
[0:57] SPEAKER_00: Not quite, but she helped create the very first vaccine.
[1:01] SPEAKER_00: Get ready, because we're diving into the epic microscopic battles happening inside you right this second, complete with giant eating cells and wanted posters for viruses.
[1:12] SPEAKER_00: So, Finn, when you get a scrape on your knee or catch a cold, how does your body know how to fix it?
[1:19] SPEAKER_01: Well, it just sort of heals.
[1:23] SPEAKER_01: I never really feel anything happening, except maybe it gets a little warm or itchy.
[1:28] SPEAKER_00: That warmth is actually a sign of an epic battle.
[1:32] SPEAKER_00: You have a built-in security team called the immune system, and its heavy hitters are your white blood cells.
[1:39] SPEAKER_00: The billions of hunters! Exactly.
[1:42] SPEAKER_00: And one of the coolest types is called a macrophage.
[1:46] SPEAKER_00: That literally translates from Greek to mean big eater.
[1:50] SPEAKER_00: Haha, big eater! How big is it really?
[1:54] SPEAKER_00: For a cell, it's absolutely massive.
[1:57] SPEAKER_00: It measures about 21 micrometers across.
[2:00] SPEAKER_00: To put that in perspective, if a normal bacteria was the size of a mouse, a macrophage would be the size of a bear.
[2:08] SPEAKER_00: Whoa, so they just chomp the germs like hungry bears?
[2:12] SPEAKER_00: They literally wrap their bodies around the germs, swallow them whole, and digest them using special chemicals.
[2:20] SPEAKER_00: But back in the late 1700s, there was a germ that multiplied so fast, even the macrophages couldn't beat it.
[2:27] SPEAKER_00: It was a virus called smallpox.
[2:30] SPEAKER_00: That sounds bad.
[2:32] SPEAKER_00: How bad was it exactly?
[2:33] SPEAKER_00: It was one of the most terrifying diseases in history.
[2:37] SPEAKER_00: It killed up to 30% of the people who caught it.
[2:40] SPEAKER_00: It was so contagious that if one person in a town got it, it would spread like wildfire.
[2:49] SPEAKER_00: That's millions and millions of people.
[2:51] SPEAKER_00: Yes, but in the English countryside, a doctor named Edward Jenner started noticing something incredibly weird about the local dairy farmers.
[3:00] SPEAKER_00: It was the year 1796, and Dr.
[3:03] SPEAKER_00: Jenner noticed that milkmaids, the women whose job was milking the cows every single day, almost never caught the deadly smallpox.
[3:12] SPEAKER_01: Why not?
[3:13] SPEAKER_01: Did the cows protect them with some kind of force field?
[3:16] SPEAKER_00: You were actually on the right track.
[3:18] SPEAKER_00: He met a milkmaid named Sarah Nelms.
[3:21] SPEAKER_00: She told him she wasn't worried about smallpox at all, because she'd already had a very mild sickness called cowpox.
[3:30] SPEAKER_01: Cowpox?
[3:31] SPEAKER_01: Is that like chickenpox, but for cows?
[3:34] SPEAKER_00: Exactly! It was a mild viral infection that gave cows little weeping glisters on their udders.
[3:41] SPEAKER_00: Sarah had caught it from her favourite Gloucester cow, whose name was Blossom.
[3:46] SPEAKER_00: She got a few spots on her hands, felt tired for a few days, and then was totally fine.
[3:52] SPEAKER_01: Wait, so getting the mild cow illness stopped her from getting the deadly human illness?
[3:58] SPEAKER_00: Dr.
[3:58] SPEAKER_00: Jenna thought so.
[4:00] SPEAKER_00: He hypothesised that cowpox somehow trained the body to fight off smallpox.
[4:05] SPEAKER_00: To prove it, he needed to do an experiment.
[4:09] SPEAKER_00: He found an eight-year-old boy named James Phipps, the son of his gardener.
[4:14] SPEAKER_01: Uh-oh, I don't like where this is going.
[4:16] SPEAKER_01: What did he do to eight-year-old James?
[4:19] SPEAKER_00: On May 14, 1796, Jenna took a tiny bit of fluid from the cowpox blister on Sarah's hand, made a little scratch on James' arm, and rubbed the fluid in.
[4:31] SPEAKER_00: He gave the kid cowpox on purpose.
[4:34] SPEAKER_00: He did.
[4:35] SPEAKER_00: Just a few days later, James felt a little achy and mildly ill, but just like Sarah, he recovered completely in about a week.
[4:44] SPEAKER_00: But here is the crazy part.
[4:46] SPEAKER_00: About six weeks later, in July, Jenna did something that would be completely illegal and against all scientific rules today.
[4:55] SPEAKER_00: He exposed eight-year-old James to the deadly smallpox virus.
[5:00] SPEAKER_00: What?
[5:01] SPEAKER_00: He gave him the actual deadly germ?
[5:03] SPEAKER_00: Did James survive?
[5:05] SPEAKER_00: He didn't even get slightly sick.
[5:08] SPEAKER_00: James was completely immune.
[5:10] SPEAKER_00: The cowpox had given his body a perfect practice run, without the danger.
[5:15] SPEAKER_01: Phew! That was a close call.
[5:17] SPEAKER_01: But how does a practice run actually work in the body?
[5:20] SPEAKER_00: Remember those white blood cells?
[5:23] SPEAKER_00: When James got the mild cowpox, his cells fought it off.
[5:27] SPEAKER_00: But some specialized cells stayed behind, as memory B cells.
[5:32] SPEAKER_00: They remembered exactly what the virus looked like.
[5:35] SPEAKER_01: So when the real smallpox showed up, they already had the battle plan.
[5:39] SPEAKER_00: Exactly! It's like giving your body a highly detailed wanted poster for the bad guys.
[5:45] SPEAKER_00: As soon as smallpox entered, the immune system destroyed it instantly.
[5:51] SPEAKER_00: It's the same reason you usually only get chickenpox once.
[5:54] SPEAKER_00: Oh I know chickenpox.
[5:56] SPEAKER_01: It makes you super itchy and covered in spots.
[5:59] SPEAKER_00: Right.
[6:00] SPEAKER_00: It's caused by the varicella zostavirus.
[6:03] SPEAKER_00: It can give a person up to 500 itchy, fluid-filled blisters all over their chest, back and face.
[6:11] SPEAKER_00: 500 blisters?
[6:13] SPEAKER_01: That is so incredibly gross.
[6:16] SPEAKER_00: It is.
[6:17] SPEAKER_00: But once your memory cells beat it, or if you get the modern chicken pox vaccine, they patrol your bloodstream for the rest of your life, making sure that virus never sneaks in and causes trouble again.
[6:30] SPEAKER_00: And that's exactly what a vaccine does.
[6:32] SPEAKER_00: It's a safe practice run for your immune system.
[6:36] SPEAKER_00: It gives you the wanted poster without you ever having to fight the real villain.
[6:41] SPEAKER_00: All thanks to Edward Jenna, Sarah the Milkmaid, and Blossom the Cow.
[6:46] SPEAKER_00: That cow is a historical hero.
[6:49] SPEAKER_00: Did Blossom get a medal or a statue?
[6:52] SPEAKER_00: Not a medal, but she got something even better.
[6:56] SPEAKER_00: She named the entire medical invention.
[6:59] SPEAKER_00: The Latin word for cow is vacca, so Jenna called the medicine a vaccine and the technique vaccination.
[7:07] SPEAKER_01: No way.
[7:08] SPEAKER_01: The word vaccine literally means cow stuff.
[7:12] SPEAKER_01: That is the coolest fact ever.
[7:14] SPEAKER_01: I'm going to tell everyone at school tomorrow.
[7:16] SPEAKER_00: I know, right?
[7:18] SPEAKER_00: And the impact of that one discovery was absolutely massive.
[7:22] SPEAKER_00: Because of vaccines, governments around the world worked together to hunt down smallpox until it was completely wiped out.
[7:30] SPEAKER_00: The last natural case in the entire world happened in 1977.
[7:36] SPEAKER_01: Wait, it's just gone.
[7:38] SPEAKER_00: Like extinct.
[7:40] SPEAKER_00: Completely gone.
[7:41] SPEAKER_00: Smallpox was eradicated from the face of the earth.
[7:45] SPEAKER_00: Vaccines have saved hundreds of millions of lives, all because a scientist got curious about why dairy farmers weren't getting sick.
[7:54] SPEAKER_01: I love that so much.
[7:56] SPEAKER_01: Science really is just noticing weird things, asking why, and then using that knowledge to save the world.
[8:02] SPEAKER_01: Alright, Finn, it is time for the quiz.
[8:05] SPEAKER_00: Are you ready to test your memory cells?
[8:08] SPEAKER_01: Okay, I've got my memory cells working at top speed.
[8:11] SPEAKER_01: I'm ready!
[8:12] SPEAKER_00: First question.
[8:14] SPEAKER_00: What was the name of the milkmaid who didn't get smallpox because she already had cowpox?
[8:23] SPEAKER_00: Next one, this is a specific detail.
[8:26] SPEAKER_00: What was the name of the eight-year-old boy in Dr.
[8:29] SPEAKER_00: Jenner's famous experiment?
[8:34] SPEAKER_00: Question three.
[8:36] SPEAKER_00: What does the Latin word vacca mean?
[8:42] SPEAKER_00: Here's number four.
[8:44] SPEAKER_00: What do we call the giant white blood cells that act like hungry bears and eat germs?
[8:53] SPEAKER_00: Last question.
[8:55] SPEAKER_00: How many white blood cells can fit in a single drop of blood the size of the head of a pin?
[9:04] SPEAKER_00: Great job! Let's see how you did.
[9:07] SPEAKER_00: The first answer is the milkmaid's name was Sarah Nelms.
[9:11] SPEAKER_00: For question two, the brave eight-year-old boy's name was James Phipps.
[9:16] SPEAKER_00: Number three, the answer is, Vacca means cow.
[9:21] SPEAKER_00: That's where the word vaccine comes from.
[9:23] SPEAKER_00: Question four.
[9:25] SPEAKER_00: Those giant germ-eating cells are called macrophages.
[9:29] SPEAKER_00: And the final answer is, you have up to 10,000 white blood cells in that tiny drop.
[9:35] SPEAKER_00: I will never look at a tiny drop of blood the same way again.
[9:39] SPEAKER_00: It's like a whole microscopic superhero movie is happening inside us every single second.
[9:47] SPEAKER_01: I just want to know if my macrophages are shaped like bears.
[9:51] SPEAKER_01: They are definitely my new favourite cells in the whole body.
[9:56] SPEAKER_00: If you loved learning about our microscopic heroes today, please leave us a five-star review.
[10:02] SPEAKER_00: Tell us if you would rather be a giant macrophage gobbling up germs or a memory cell holding the wanted poster.
[10:09] SPEAKER_01: Just scroll down on your podcast app and tap the five stars.
[10:14] SPEAKER_01: It really helps other kids find the show.
[10:17] SPEAKER_00: Oh, and don't forget to check out our site kidopoly.com.
[10:21] SPEAKER_00: We've got tons of fun learning games and activities there.
[10:24] SPEAKER_00: It's the best place to keep exploring.
[10:27] SPEAKER_01: Yeah, and if you want a shout out on the show or just want to say hi, email us at hello at kidopoly.com.
[10:35] SPEAKER_01: We love hearing from you.
[10:37] SPEAKER_00: Thanks for exploring the amazing world inside your body with us today.
[10:41] SPEAKER_00: See you next time on Science is not boring.
[10:44] SPEAKER_00: Bye everyone.
[10:46] SPEAKER_00: Stay curious!

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