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00:00Moby, is that you? I just wanted to tell you, you're the best friend I ever had.
00:12Oh, sure, I'm up for a letter.
00:17Dear Tim and Moby, why do we get sick? From Ashley.
00:22Yeah, I was just asking myself the same question.
00:25No one likes to have a fever or a cough that just won't stop.
00:30But you'd get sick a lot more if it weren't for your immune system.
00:36That's a system of cells, tissues, and organs that help your body fight off diseases.
00:42It starts with some general defenses against pathogens.
00:45Those are substances or organisms that can make you sick.
00:51Including viruses, like those that cause colds and flus.
00:55Bacteria, which cause diseases like tetanus.
00:58Fungi, the cause of ailments like athlete's foot.
01:01Or parasites, such as the microscopic critters that cause malaria.
01:06Skin acts as a barrier, preventing pathogens from entering your body.
01:10Mucus in your nose traps many pathogens you inhale, stopping them from getting into your bloodstream.
01:15And stomach acids kill bacteria that live on the food you eat.
01:19These frontline defenses are part of our innate immune system.
01:23They're sort of like shields, because they defend against everything the same way, no matter what it is.
01:29Sometimes, though, germs get by those defenses and into your blood.
01:33Luckily, cells in the immune system patrol the bloodstream for these invaders.
01:38If these white blood cells, or leukocytes, recognize a germ, they'll kill it before you get sick.
01:44Your immune system sends a lot of germs running for their lives on a daily basis.
01:49But if it doesn't recognize them, the germs could invade your cells, multiply, and make you sick.
01:55That's an infection, and it's where your adaptive immune system kicks in.
01:59It works by learning how to identify and destroy specific pathogens.
02:04After a while, white blood cells usually figure out what's making you sick.
02:08To fight the invaders, they create specialized weapons called antibodies.
02:13Each one's designed to lock onto an antigen, a specific piece of the invading germ's skin.
02:19The antibody is sort of a red flag, signaling leukocytes to kill the germ.
02:24And some antibodies can neutralize germs on their own.
02:29Your immune system can fight off most infections.
02:32But sometimes it needs a little help.
02:35Like from antibiotics, medicines that kill bacteria.
02:38Or from vaccines, which protect your body against bacteria and viruses.
02:43Oh, and the coolest thing about your immune system is that it remembers.
02:47With many diseases, like the chickenpox, your body can use the same antibodies to fight it off next time.
02:55If that pathogen tries to enter your body again, the immune system will know how to get rid of it.
03:00That's how vaccines work.
03:02They train your immune system to make antibodies without getting you sick.
03:07Most vaccines are shots that inject a tiny amount of the virus or bacteria.
03:12Usually, vaccines use a killed or weakened form of the pathogen, or just a piece of it.
03:18But your immune system still flags it as an invader and creates antibodies for it.
03:23So if those germs ever come your way again, they won't make it far.
03:28Once you have the antibodies, you're resistant, or immune, to that pathogen.
03:35Well, your immune system doesn't always work the way it's supposed to.
03:39Simple things like not getting enough sleep or not eating right can weaken it.
03:43Unhealthy habits like drug abuse can do the same.
03:46Any of these behaviors might cause a person to get sick more often.
03:50In some people, the immune system goes haywire from a condition called autoimmunity.
03:55The immune system attacks healthy cells in the body as if they're invaders.
04:00In rare cases, pathogens weaken the immune system to the point that it can't do much at all.
04:06You've probably heard of HIV. It stands for Human Immunodeficiency Virus.
04:11HIV attacks white blood cells and weakens the immune system to the point that it can't do its job.
04:17This illness is called AIDS, or Acquired Immune Deficiency Syndrome.
04:22Something like a simple cold can make a person with AIDS really sick.
04:27Right, even healthy people can get the common cold over and over again.
04:32That's because it's actually lots of different viruses that produce similar symptoms.
04:37Just because your immune system fought off one cold virus doesn't mean it'll be ready for the next one.
04:43Uh, Moby, I need a tissue.
04:48Please? I'm weak.