Jugami › Life Science › Disease and Immunity

Life Science · Grades 6–8

Disease and Immunity

Germs, the body’s defenses, and vaccines.

Germs are all around us, in the air, soil, and water, and on our skin. Most of them do us no harm, and some even help keep us healthy. A few can make us sick. Luckily, your body has a whole team of defenses that works every day, mostly without you noticing.

On this page you will meet the four main kinds of germs, see how they spread, and follow your body’s three lines of defense. You will also learn how vaccines work, why antibiotics do not help a cold, and how to wash your hands well. Then try Barrier, Innate, or Adaptive? and the six-question quiz.

Play Barrier, Innate, or Adaptive? Take the quiz

Four kinds of germs

A germ that causes disease is called a pathogen. The living thing it invades is the host. Most pathogens are microorganisms, so small you need a microscope to see them. The main kinds are:

Bacteria are one-celled living things. Most are harmless or helpful, but a few cause illness, such as strep throat.
Viruses can copy themselves only inside the cells of a living host. Colds and the flu are caused by viruses.
Fungi can cause infections too. Athlete’s foot is a common skin infection caused by a fungus, usually between the toes.
Protists are a varied group of mostly one-celled living things. Malaria is caused by a protist called Plasmodium, which spreads to people through the bites of some kinds of mosquitoes.

Four kinds of pathogens, with one example of each. The drawings are not to scale.

How germs spread

An infection begins when a pathogen gets into a host and starts to multiply. Germs can spread from person to person in several ways:

by touching someone who has the germ, or sharing cups or forks;
by breathing in air after someone with the germ coughs or sneezes;
by touching an object or surface that has germs on it, then touching your eyes, nose, or mouth;
through food or water, or through the bite of an animal such as a mosquito or tick.

Not every germ spreads every way. Malaria, for example, does not pass from person to person; it needs a mosquito.

Three lines of defense

1. Barriers. Unbroken skin is a wall that most germs cannot get through. It is dry and slightly acidic, which kills or slows many germs. Where there is no skin, as in your eyes, nose, and throat, tears and sticky mucus trap and rinse germs away, and tiny hairs called cilia sweep mucus out of your airways. Acid in your stomach slows germs that you swallow.

2. The innate response. If germs get past the barriers, for example through a cut, the innate immune system acts fast. It attacks many kinds of germs in the same general way. White blood cells rush to the spot and engulf germs, swallowing and breaking them down. The area may become red, warm, and swollen; this is called inflammation. Sometimes the body also raises its temperature, causing a fever, which can slow some germs.

3. The adaptive response. If the innate response is not enough, the adaptive immune system steps in. It is slower to start, taking days or even weeks, but it targets one exact germ and remembers it.

The body’s three lines of defense. Based on OpenStax Biology 2e, chapter 42.

Antibodies and memory

The adaptive system uses two main kinds of white blood cells, B cells and T cells. Some T cells find and destroy body cells that a virus has infected. B cells make antibodies, Y-shaped proteins that fit one kind of germ, like a key in a lock. Antibodies can coat germs so they cannot enter cells, and they make it easier for other white blood cells to engulf them.

After the germ is gone, some B and T cells stay behind as memory cells. If the same germ comes back, the memory cells recognize it and the body responds faster and more strongly.

Antibodies after a first and a second meeting with the same germ. A sketch of the general shape, not real data.

Vaccines

A vaccine trains the immune system before you meet a germ. Many vaccines contain killed or weakened forms of a germ so they do not cause the disease. Your body makes antibodies and memory cells anyway. Later, if the real germ shows up, your immune system is ready. Immune memory is the reason vaccines work.

Some vaccines need a booster, an extra dose given later, to keep memory strong. Most vaccines are given as a shot, but some are taken by mouth or sprayed into the nose.

Antibiotics and viruses

Antibiotics are medicines that treat some infections caused by bacteria. They do not work on viruses, so they will not help with a cold or the flu. Even some bacterial infections get better without them. Only a doctor or other health care professional can decide whether someone needs an antibiotic.

Washing your hands

Washing your hands with soap and water is one of the best ways to stay healthy and stop germs from spreading. The CDC lists five steps:

Wet your hands with clean, running water, warm or cold. Turn off the tap and apply soap.
Lather by rubbing your hands together with the soap, including the backs of your hands, between your fingers, and under your nails.
Scrub for at least 20 seconds. Need a timer? Hum “Happy Birthday” from beginning to end twice.
Rinse well under clean, running water.
Dry with a clean towel or an air dryer.

Good times to wash include before eating, after using the toilet, and after blowing your nose, coughing, or sneezing. If soap and water are not around, the CDC says a hand sanitizer with at least 60% alcohol can be used.

Key words

Pathogena germ, usually a microorganism, that causes disease
Hostthe living thing a pathogen invades
Infectionwhen a pathogen gets into a host and multiplies
Innate immunityfast, general defenses that attack many kinds of germs
Adaptive immunityslower defenses that target one exact germ and remember it
White blood cella blood cell that fights infection
Antibodya Y-shaped protein made by B cells that fits one kind of germ
Memory cella B or T cell that remembers a germ and responds fast next time
Vaccinesomething that trains the immune system without causing the disease
Antibiotica medicine that treats some infections caused by bacteria

Game: Barrier, Innate, or Adaptive?

You will see twelve clues, one at a time. Decide which kind of defense each one describes: a barrier, the innate response, or the adaptive response. A clue you sort on your first try earns 1 point, for a total of 12. Your best score is saved on this device.

The game needs JavaScript. You can still check every clue in the table below.

Work through it without the game. Cover the two right-hand columns, name the kind of defense for each clue, then check.

Barrier, Innate, or Adaptive?: all twelve clues
Clue	Defense	Why
Unbroken skin dries out germs and is too acidic for many of them	Barrier	Skin is the first wall that keeps germs from getting inside the body.
Sticky mucus in your nose traps germs before they go deeper	Barrier	Mucus is a physical barrier that traps germs and helps rinse them away.
Tiny hairs in your airways sweep mucus and trapped germs back out	Barrier	Cilia push germ-filled mucus out of the body, so germs never get a foothold.
Strong acid in the stomach slows germs swallowed with food	Barrier	The low pH of the stomach is a chemical barrier against germs.
A white blood cell swallows a bacterium whole and breaks it down	Innate	Engulfing germs is a fast, general defense that attacks any invader.
A cut turns red, warm, and swollen as white blood cells rush in	Innate	That is inflammation, part of the fast, general innate response.
Body temperature rises during an infection	Innate	A fever is part of the innate response and can slow some germs.
First responders attack any invader, not one exact kind	Innate	The innate system acts quickly against many germs but does not target one kind.
Y-shaped proteins lock onto one exact germ	Adaptive	Antibodies are made by B cells and fit only one kind of germ.
Cells remember a germ and respond faster the next time	Adaptive	Memory cells are why the adaptive system protects you on a second exposure.
A vaccine trains the body to make antibodies without causing the disease	Adaptive	Vaccines build immune memory so you do not have to get sick first.
A defense that takes days or weeks to build but targets one germ	Adaptive	The adaptive response is slower to start, but it is specific and has memory.

Quick quiz

Six questions. Pick one answer for each, then press Check my answers.

1. Which kind of germ can copy itself only inside a living cell? A bacterium A virus A fungus

2. Which of these is a barrier that keeps germs out? A memory cell An antibody made by a B cell Sticky mucus in the nose

3. How is the adaptive response different from the innate response? It works within minutes on any germ It targets one germ and remembers it It only works on the skin

4. Why does the body fight a germ faster the second time? Memory cells remember it The germ gets weaker each year Skin grows thicker after an illness

5. Why will antibiotics not help a cold? Colds are caused by viruses, and antibiotics do not work on viruses Colds go away too fast Antibiotics only work on fungi

6. How long does the CDC say to scrub your hands with soap? About 5 seconds At least 20 seconds Exactly one minute, with hot water only

Answer key: 1. A virus. 2. Sticky mucus in the nose. 3. It targets one germ and remembers it. 4. Memory cells remember it. 5. Colds are caused by viruses, and antibiotics do not work on viruses. 6. At least 20 seconds.

Talk about it

Which of your body’s barriers do you think works hardest on a normal school day? Why?
Why might it help the body to have both a fast, general defense and a slower, exact one?
How is a vaccine like a fire drill for your immune system?
A friend has a cold and wants an antibiotic. What would you tell them, and who should they ask?
Where at school would a reminder to wash hands do the most good?

Sources and credits

OpenStax, Biology 2e, Chapter 42 Introduction (pathogens are agents, usually microorganisms, that cause disease in a host; they include bacteria, protists, fungi, and other infectious organisms).
OpenStax, Biology 2e, 42.1 Innate Immune Response (skin as a barrier, drying and acidity; tears, mucus, and cilia; stomach acid; all viruses replicate inside cells; engulfing of pathogens, inflammation, and fever).
OpenStax, Biology 2e, 42.2 Adaptive Immune Response (adaptive immunity takes days or weeks and has memory; T cells and B cells; memory cells; immune memory is the reason vaccines work; boosters).
OpenStax, Biology 2e, 42.3 Antibodies (Y-shaped structure; antibodies neutralize pathogens by blocking sites they use to enter cells, and help engulfing).
OpenStax, Biology 2e, 21.1 Viral Evolution, Morphology, and Classification (most virions are about 20 to 250 nanometers across and, apart from large ones, cannot be seen with a light microscope; early filters removed all bacteria but not viruses).
OpenStax, Biology 2e, 23.4 Ecology of Protists (Plasmodium, the protist that causes malaria).
NIH, MedlinePlus, Athlete’s Foot (a common infection caused by a fungus, most often between the toes).
CDC, About Handwashing (how germs spread from hands; key times to wash; the five steps; at least 20 seconds; hand sanitizer with at least 60% alcohol).
CDC, Antibiotic Use: About (antibiotics treat certain bacterial infections such as strep throat; they do not work on viruses such as those that cause colds and flu).
WHO, Vaccines and immunization: What is vaccination? (vaccines train the immune system to make antibodies; killed or weakened germs do not cause the disease; shots, by mouth, or nasal spray).
WHO, Malaria fact sheet (spread to people by some types of mosquitoes; does not spread from person to person).
NIH, National Institute of Allergy and Infectious Diseases, Overview of the Immune System (skin as the first line of defense; innate and adaptive immune cells; B and T cells and immunological memory; white blood cells).
NIH, MedlinePlus, Germs and Hygiene (germs are everywhere and many are harmless or helpful; the main types of germs; the ways germs spread).

Credits

Text, game, and quiz written by Dian, Life Science teacher at Jugami. All diagrams are original, hand-coded SVG drawings made for this page, and the share image was drawn in code. The diagrams are simplified and not to scale.