disease and infection
protecting the body
This entire unit, we’ve talked about how all of the parts of the body work in perfect harmony to create the marvel that is you. Your heart beats in perfect time. Your lungs bring in much-needed oxygen that is then delivered seamlessly to your cells. Cells communicate exactly what they need with one another via your nervous and endocrine system, all working to keep everything in the perfect homeostatic balance for life. You even have mechanisms to protect you from the things that threaten that balance, like your reflexes that save you from burning your hand on a hot stove, or your fight-or-flight response that puts your body into superhero-mode and allows you to evade bear attacks and scary tests. But what do we do when that threat is much, much smaller? How do we protect ourselves from the invaders we can’t see but fill every inch of the world around us: the viruses and bacteria? And what about microscopic threats that emerge from within, like cancer? Enter: Immune System.
the nature of disease
As cool as your body is and as impressive as it is that it can keep so many complex systems running effectively most of the time, you probably already know that it’s not perfect, even barring any attacks from outside sources. There are definitely a lot of tiny mistakes that can happen in such a complex system. Proteins can fold wrong and not be able to do their job right. They can even fold wrong and start doing harmful things, like aggregating into clumps. DNA can become damaged or replicate incorrectly, causing widespread errors in the activity of a specific protein or even a whole cell. Membranes can become damaged, nutrients can become sparse, cells die. Disease can emerge in many places.
But, our bodies have lots of fail-safes and redundancies in place to make sure that a tiny mistake doesn’t kill you. We have proteins that monitor other proteins and destroy them if they’re not made properly. We have proteins that monitor and repair DNA. We have checkpoint inhibitors that stop cells from growing out of control. We have pathways that cause old or damaged cells to die before they become damaging. Disease can still emerge, especially if any of these pathways are broken, but our bodies are constantly hard at work to keep you in perfect balance and to keep any tiny mistake from killing you.
The immune system is one of the key players in protecting our body against disease. It constantly monitors every crevice and corner of your body looking for anything that looks suspicious. And that’s not just foreign invaders! (Though this is one of the most prominent aspects of the immune response). When the immune system encounters a dying cell, a cancerous or precancerous one, or anything that doesn’t look like the “normal” cells that your immune system has been trained to tolerate, immune cells leap into action and attack. We’ll learn more about the specifics of this attack in the next part of this lesson.
But, our bodies have lots of fail-safes and redundancies in place to make sure that a tiny mistake doesn’t kill you. We have proteins that monitor other proteins and destroy them if they’re not made properly. We have proteins that monitor and repair DNA. We have checkpoint inhibitors that stop cells from growing out of control. We have pathways that cause old or damaged cells to die before they become damaging. Disease can still emerge, especially if any of these pathways are broken, but our bodies are constantly hard at work to keep you in perfect balance and to keep any tiny mistake from killing you.
The immune system is one of the key players in protecting our body against disease. It constantly monitors every crevice and corner of your body looking for anything that looks suspicious. And that’s not just foreign invaders! (Though this is one of the most prominent aspects of the immune response). When the immune system encounters a dying cell, a cancerous or precancerous one, or anything that doesn’t look like the “normal” cells that your immune system has been trained to tolerate, immune cells leap into action and attack. We’ll learn more about the specifics of this attack in the next part of this lesson.
pathogens
Now that we’ve established that the immune system is involved in more than just protecting against foreign invasion and that there are more mechanisms than just the immune system that protect you from various kinds of diseases, let’s talk more about outside threats.
Microorganisms are absolutely everywhere on this planet. There are even microorganisms that can live in extreme environments like volcanoes and hot springs. Many of these microorganisms are harmless species just fighting for life. Many of them are even beneficial to life, providing distant help to human life like recycling nitrogen to keep the environment stable or very direct help to human life like living in our guts and digesting the nutrients that we can’t. Other microorganisms are harmful to us. None are sentient—it’s not like they know they’re attacking a person or care what they’re growing on—but all have adapted and are continuing to very rapidly evolve new mechanisms to better stay alive, which sometimes means hijacking humans.
We’ll focus mainly on the pathogenic microorganisms--pathogens for short—that threaten humans. There are four main types:
Viruses
We’ve briefly mentioned viruses before, when we were talking about what it means for something to be alive. Viruses are not alive, but they do a pretty good job of mimicking life, all the same. They are little bits of nucleic acid (sometimes DNA, sometimes just RNA, either of which can be double-stranded or single-stranded) encapsulated in a casing of proteins and sometimes a phospholipid “envelope” (it’s not nearly as advanced as your cell membrane, so we don’t call it that). They don’t do much of anything unless they’ve infected a cell. Once they’re in a cell, though, they use the cell’s protein-making machines to make the proteins encoded by their own nucleic acids. These proteins are mainly ones that allow the virus to reproduce, escape the cell, and go on to infect other cells, so that the virus can keep spreading. The better a virus is at spreading, the more abundant it will become in our environment, and the more likely it is that we will acknowledge it as a threat. Because they’re such simple almost-organisms, they also mutate and evolve much more rapidly than larger and more complex systems.
This video talks more about the general structure and not-quite-life cycle of a virus:
Microorganisms are absolutely everywhere on this planet. There are even microorganisms that can live in extreme environments like volcanoes and hot springs. Many of these microorganisms are harmless species just fighting for life. Many of them are even beneficial to life, providing distant help to human life like recycling nitrogen to keep the environment stable or very direct help to human life like living in our guts and digesting the nutrients that we can’t. Other microorganisms are harmful to us. None are sentient—it’s not like they know they’re attacking a person or care what they’re growing on—but all have adapted and are continuing to very rapidly evolve new mechanisms to better stay alive, which sometimes means hijacking humans.
We’ll focus mainly on the pathogenic microorganisms--pathogens for short—that threaten humans. There are four main types:
Viruses
We’ve briefly mentioned viruses before, when we were talking about what it means for something to be alive. Viruses are not alive, but they do a pretty good job of mimicking life, all the same. They are little bits of nucleic acid (sometimes DNA, sometimes just RNA, either of which can be double-stranded or single-stranded) encapsulated in a casing of proteins and sometimes a phospholipid “envelope” (it’s not nearly as advanced as your cell membrane, so we don’t call it that). They don’t do much of anything unless they’ve infected a cell. Once they’re in a cell, though, they use the cell’s protein-making machines to make the proteins encoded by their own nucleic acids. These proteins are mainly ones that allow the virus to reproduce, escape the cell, and go on to infect other cells, so that the virus can keep spreading. The better a virus is at spreading, the more abundant it will become in our environment, and the more likely it is that we will acknowledge it as a threat. Because they’re such simple almost-organisms, they also mutate and evolve much more rapidly than larger and more complex systems.
This video talks more about the general structure and not-quite-life cycle of a virus:
Some viruses have very clever proteins that allow them to reproduce better by causing different types of symptoms in their host that allow them to survive longer and spread to more people. Again, the viruses aren’t actually strategizing, but the mechanisms of natural selection (which we’ll talk more about in our next unit) that allow the viruses that are best at reproducing to become the most prolific can sometimes be so significant that it’s hard not to think of some viruses as master strategizers. For example, think of all of your common cough and cold viruses: They cause irritation to the airways and lungs, causing you to cough and sneeze and that’s what spreads them! Herpes virus hides out in your nerves, which are immunoprotected (not likely to be attacked by your immune system), until they think that you are dying (so, when your fight-or-flight system gets activated, like when you’re being chased by a bear, or are worried about a test), at which point they cause cold sores that go out and infect other people. Ebola causes massive hemorrhaging (bleeding out) and is spread through the blood. Almost all viruses that are any real threat to us have some mechanism of fighting back against the immune system, possibly the most prominent example of which is HIV, which completely shuts down your immune response if untreated. If you’re interested in learning more about any of these viruses or others, check out some of these videos:
How a Sick Chimp Led to a Global Pandemic: The Rise of HIV
What we know (and don't know) about Ebola
Why Spanish Flu Killed Over 50 Million People - The Deadliest Plague in Modern History
What is Coronavirus (Covid-19)?
How do viruses jump from animals to humans?
How pandemics spread
Why Is The Measles Virus So Contagious?
What we know (and don't know) about Ebola
Why Spanish Flu Killed Over 50 Million People - The Deadliest Plague in Modern History
What is Coronavirus (Covid-19)?
How do viruses jump from animals to humans?
How pandemics spread
Why Is The Measles Virus So Contagious?
Fortunately, our immune systems are also pretty good at fighting back against viruses, and we smart humans with our brains have found some clever ways of making our immune systems even better. For example, we’ve developed a much better understanding of disease over the course of history, which has led to robust improvement in our systems of public health. We’ve also eradicated some of history’s most deadly diseases through vaccination, in which we train our immune system to recognize an especially deadly threat so that we can eliminate it immediately the next time that we see it, instead of giving it time to grow and infect us. We’ll learn more about how this is possible in the next part of this lesson. Here’s an example of just one of many deadly diseases that has been eradicated through public health and vaccination:
For more on vaccines, check out this video:
Bacteria
Bacteria are simple, unicellular organisms. They can come in many shapes and forms, and some have unique features that others don’t. For example, they can be round, rod-like, or even spiral in shape. Some have flagella, which are like little tails that help them to swim through water or infect your cells more effectively. We’ll learn more about all types of bacteria later on. For now, we’ll focus on how they can cause disease. Much like viruses, they aren’t really trying to infect you, but the ones that are most successful at reproduction are the ones that become most abundant in the world, and, when they do this by causing harm, that’s a big problem for us. This video talks more about how bacteria cause disease:
Bacteria are simple, unicellular organisms. They can come in many shapes and forms, and some have unique features that others don’t. For example, they can be round, rod-like, or even spiral in shape. Some have flagella, which are like little tails that help them to swim through water or infect your cells more effectively. We’ll learn more about all types of bacteria later on. For now, we’ll focus on how they can cause disease. Much like viruses, they aren’t really trying to infect you, but the ones that are most successful at reproduction are the ones that become most abundant in the world, and, when they do this by causing harm, that’s a big problem for us. This video talks more about how bacteria cause disease:
Like viruses, they have a lot of interesting mechanisms for fighting back against our immune system, which can sometimes be more advanced than viral mechanisms because bacteria are living and are more complex than viruses. Here are a few examples of bacteria that you may be interested in learning more about:
What Made the Black Death (The Plague) so Deadly?
What Makes Tuberculosis (TB) the World's Most Infectious Killer?
The Surprising Cause of Stomach Ulcers
The Story of Cholera
What E. Coli Does To Your Body
What is Botulism?
What Makes Tuberculosis (TB) the World's Most Infectious Killer?
The Surprising Cause of Stomach Ulcers
The Story of Cholera
What E. Coli Does To Your Body
What is Botulism?
The history of how we came to understand that many diseases come from bacteria is pretty fascinating. Here’s more on that, if you’re interested:
Humans have also gotten pretty good at fighting bacteria using antibiotics, which are basically bacteria-specific poisons that hurt bacteria but not you. Unfortunately, bacteria evolve so quickly that many have already found ways to avoid getting killed by antibiotics, so we need to continue finding new and better ways to fight against bacterial diseases. This video explains more on that:
Fungi
Fungi is a type of eukaryotic cell, but in a totally different family from plants or animals. The most common fungus that people tend to be familiar with is mushrooms. But, just like bacteria can make delicious cheese and yogurt or nasty, devastating diseases, fungus can be pretty gross. Fungal infections can range from relatively minor (but still no fun) athlete’s foot, ringworm, and yeast infections (candidiasis) to potentially life-threatening ones like histoplasmosis and aspergillosis, which are both lung infections.
Fungi is a type of eukaryotic cell, but in a totally different family from plants or animals. The most common fungus that people tend to be familiar with is mushrooms. But, just like bacteria can make delicious cheese and yogurt or nasty, devastating diseases, fungus can be pretty gross. Fungal infections can range from relatively minor (but still no fun) athlete’s foot, ringworm, and yeast infections (candidiasis) to potentially life-threatening ones like histoplasmosis and aspergillosis, which are both lung infections.
Fungal infections grow for the same reason as any other infections: These fungi are just trying to stay alive, and you’re a very abundant source of nutrients. Fungal infections don’t typically grow quite as fast or mutate as quickly as viral and bacterial infections, because, as eukaryotes, they’re bigger and more complex. But, because they look more similar to your cells than viruses and fungi do, it’s harder to target and kill them specifically. This includes both immune attack and attack by therapeutics. So, our current best antifungal medications are still pretty unpleasant and cause a lot of side-effects to your cells.
Protozoa and Parasites
Protozoa are single-celled eukaryotes. You may have heard them called protists. Amoeba are one example of a non-pathogenic protozoa. The pathogenic protozoa include malaria, Chagas disease, and giardia (a common type of diarrhea caused by drinking contaminated water). They are often lumped in the same category as parasites, which consist of the multicellular organisms that directly cause disease, like intestinal worms and the bugs that cause scabies.
Parasites often transmit through non-human hosts. For example, malaria transmits through mosquitos, and another parasite called toxoplasmosis transmits through cats. If you’re interested in learning more about some protozoan and parasitic diseases, check out these videos:
Protozoa and Parasites
Protozoa are single-celled eukaryotes. You may have heard them called protists. Amoeba are one example of a non-pathogenic protozoa. The pathogenic protozoa include malaria, Chagas disease, and giardia (a common type of diarrhea caused by drinking contaminated water). They are often lumped in the same category as parasites, which consist of the multicellular organisms that directly cause disease, like intestinal worms and the bugs that cause scabies.
Parasites often transmit through non-human hosts. For example, malaria transmits through mosquitos, and another parasite called toxoplasmosis transmits through cats. If you’re interested in learning more about some protozoan and parasitic diseases, check out these videos:
How parasites change their host's behavior
Malaria is the deadliest disease in human history
Toxoplasmosis: How Parasites in Your Cat Can Infect Your Brain
How Worms Survive Inside Your Body
SCABIES, Causes, Signs and Symptoms, Diagnosis and Treatment
Malaria is the deadliest disease in human history
Toxoplasmosis: How Parasites in Your Cat Can Infect Your Brain
How Worms Survive Inside Your Body
SCABIES, Causes, Signs and Symptoms, Diagnosis and Treatment
If the immune system is so good, why do we still get sick?
The fastest way to summarize everything I just said about different pathogens is that there are lots of things out in the world that could potentially hurt you. The world is filled with all kinds of microorganisms. In fact, in a single drop of seawater, there are somewhere around 10 million viruses, 1 million bacteria, and 1,000 protozoa! (Who really runs the world?!) While only a small fraction of these are pathogenic, that’s still an awful lot of pathogens in the world. Considering that the average person only gets a common cold perhaps a couple of times a year and many people will only contract a more serious infectious disease perhaps once or twice in their whole lives, that must mean our immune system does a pretty good job of fighting back against the tiny emperors of the world.
But, our immune systems do still fail us from time to time. We still get sick. One big part of this is that pathogens—especially viruses and bacteria—are good at fighting back against the immune system. They can evolve mechanisms to sneak around immune attack, or they can find ways to directly shut down the immune response. But, your immune system is good, too. It gets even smarter to fight back in new and better ways. It’s kind of like one big cellular chess match, playing by the rules of natural selection. For all the good our immune system does us and every fight it wins, it’s hard to blame it for losing the occasional match.
But, our immune systems do still fail us from time to time. We still get sick. One big part of this is that pathogens—especially viruses and bacteria—are good at fighting back against the immune system. They can evolve mechanisms to sneak around immune attack, or they can find ways to directly shut down the immune response. But, your immune system is good, too. It gets even smarter to fight back in new and better ways. It’s kind of like one big cellular chess match, playing by the rules of natural selection. For all the good our immune system does us and every fight it wins, it’s hard to blame it for losing the occasional match.
Summary
You should understand:
If you’re interested in learning more about the therapeutics that we use to attack different pathogens, this video sums them up nicely:
- That the immune system is involved in protecting against both infectious and some non-infectious diseases, like cancer.
- The main similarities and differences among the 4 main types of pathogens: viruses, bacteria, fungi, and protozoa and parasites.
- That the less complex a pathogen is, the faster it can evolve new defenses against the immune system.
- That the more similar a pathogen is to human cells, the harder it is for your immune system or medicine to target specifically and kill.
If you’re interested in learning more about the therapeutics that we use to attack different pathogens, this video sums them up nicely:
Learning Activity
Content contributors: Emma Moulton and Emily Zhang