Introduction: “Hi, I’m a Microbe”
The first week of the fall semester at Fairview is interrupted by construction, as a work crew jackhammers a water line through the concrete base on the eastern wing. Sometime over the summer, the system had gotten clogged by mineral deposits. The small blockage had quickly become an immense problem when slurry of art supplies had been poured down the drain and become trapped at the bottleneck created by the accumulated salt. Since the system had been covered over by concrete during a poorly-planned former renovation, the entire area was now torn up for the reconstruction. The teachers, driven mad by the constant noise, had cancelled several classes already. Mrs. K, with her sensitive ears, was no exception.
As a result, the second week of school is well under way before the four classmates experience a full period of instruction, rather than long reading assignments in their (numerous) textbooks. On this day, Mrs. K (late again) begins talking non-stop as she enters, throwing her bag down quickly on the desk.
“Good afternoon, did everyone get the assignment for today?”
They nod, and Angelica and Maxine exchange hesitant looks – they’re not quite sure what they think of today’s activity, but if past experience is any indication, it’s probably going to be 1) embarrassing and 2) not particularly educational. Mrs. K turns to the class.
“Who wants to go first?”
Their assignment had been to look up information on a specific microbe and present it to the class in an “interview”; they would pretend to be the organism, and their classmates would ask questions about it.
No one raises their hand.
“Fallon,” says Mrs. K, hoping to break the awkward silence, “why don’t you begin?”
Sensing that fate has not been kind to him today, Fallon slowly pulls out his scribbled notes on tuberculosis, trying to run through the fact in his mind one more time. Having retrieved his outline, he slouches back in his chair, waiting for the questions to begin.
“Would you mind coming to the front of the room, so we can all see you?” asks Mrs. K. Fallon quickly complies, growing more nervous as he turns to face his (clearly skeptical) classmates.
“And who are you?” asks Mrs. K.
“Oh, hi, I’m Fallon,” he answers.
“No . . .,” she says, “what microbe are you.”
The other students giggle, clearly unimpressed with his introduction. “Hi, I’m Lang – I don’t believe we’ve met,” whispers Lang, drawing more laughter from the two girls. Fallon sighs, sensing this isn’t going to be pleasant.
“Hi, I’m tuberculosis, and I’m a pathogen.”
“What to do you mean, you’re a pathogen?” asks Mrs. K. Fallon stares blankly at her, trying to think of an answer.”
“Wikipedia says I’m a pathogen.”
The laughter from the other students is louder this time, causing Mrs. K to glare in their direction. They quiet, as she turns again to Fallon. “So what is a pathogen? Why aren’t you just a microorganism? Are all microbes pathogens?”
Fallon, finally understanding, answers, “oh, because I can make people sick.” This, too, his classmates find quite hilarious. “I’m contagious,” he adds, which doesn’t quiet their laughter.
“Right,” says Mrs. K, “a pathogen is a microbe that can cause disease. There are a lot of tiny organisms that don’t – in fact, most don’t. It’s just the ones that can harm us that are called pathogens.”
“Why do you make people sick?” Angelica asks Fallon, trying to keep a straight face.
“Yes,” says Mrs. K, “good question. Why do you cause an infection?” Fallon tries to remember what Wikipedia said about this, and pulls blank. Maxine raises her hand to answer, and despite the fact that this is his presentation, Mrs. K calls on her – no point in prolonging this, after all.
“The tuberculosis bacterium,” she starts, correcting Fallon’s mistake, “can cause an infection because it can get inside the human body and grow, if the environment is right – I think the tuberculosis bacterium usually grows in the lungs.”
“Good, says Mrs. K.” Maxine continues.
“But the bacterium doesn’t cause the disease tuberculosis unless the fact that is growing actually harms the body. Even Wikipedia,” she smirks, “would probably tell you that there are a lot of people with the bacterium growing inside of them who never develop the disease.”
“Very good,” says Mrs. K. “So an infectious disease is . . .”
“A disease caused by microorganisms invading the human body and damaging it by growing and killing cells and tissues in that body,” answers Angelica. Now Fallon is mad – her answer was on Wikipedia, he remembers the exact line.
“Lang,” asks Mrs. K, “would you like to go next.”
“I don’t understand the point of this assignment,” he answers.
“Well, by role-playing different microbes,” she replies, “you’re supposed to all look up the information that you’re supposed to be reading. Plus, this is interactive, so you learn it again in a different way.”
“Sure,” he replies, “but microbes don’t talk. What is the point of me playing a talking virus when talking viruses don’t exist?”
She starts to think of a reply, but stops, deciding to end this experiment for the day. “Very well, I think we’ve had enough acting for today. Everyone just turn in a three page report on your pathogen next time. Class dismissed.”
The Major Themes of this Course:
Throughout Superbugs, Science, & Society, we will revisit the same concepts again and again. Sometimes the specific details of a given topic may seem overwhelming, but they should be clearer if you keep a few basic principles about infectious disease in mind:
1. Pathogens are Unfriendly Microbes
It’s a testament to the diversity of life that a single drop of lake water or a handful of dirt can contain a rich ecosystem of tiny organisms. Most of these are harmless to us, and many are actually helpful, as you’ll find out in our discussion of bacteria. The fact that we don’t get sick every time we go outside should be proof that only a limited range of microbes can cause disease: we call these pathogens. Pathogenic microbes include bacteria, viruses, fungi, protozoa (single-celled organisms like the trypanosomes we’ll discuss later), prions, and helminths (multicellular organisms). We’ll see all but the last group in this course, and provide more information as it’s necessary.
How do these pathogens cause disease? Like any organism, microbes want to grow, multiplying and spreading out into new places. They are infectious if the human body is one of the places they can grow; however, they only cause disease if they are destructive to their home, killing the tissues they inhabit.
2. How do we know a Pathogen Causes a Disease?
In the days before modern science, people used to think diseases were caused by all sorts of strange things – swamp gas, bad blood, or evil spirits. The idea that tiny creatures were actually responsible was a major breakthrough. The proof is summarized by “Koch’s Postulates,” a set of four rules that establish how a microorganism can be nailed as the culprit of a disease.
1. The microbe is found in the tissues of anyone with the disease.
2. The microbe can also be isolated from the infected host and grown in pure culture.
3. A healthy host will develop the disease if injected with this pure culture of the microbe.
4. The microbe must also be able to be isolated from this second, experimentally infected host.
We’ll return to these rules many times, and even look at some situations where they don’t apply. In general, though, they serve as a good rule of thumb.
3. Pathogens have Multiple Ways to Jump Ship
How does a microbe get inside our bodies to begin with? In general, pathogens can be acquired by direct, indirect, or vertical transmission.
Direct transmission is probably most familiar to us. Whether a fellow patient sneezes on you at the doctor’s office, or, in a more extreme case, if you are bitten by a rapid dog, disease causing microbes directly enter the body through saliva, blood, mucus, or skin contact. We say this is direct transmission because the microbe has directly moved between its former location – the “reservoir” – to the new one – you.
Indirect transmission occurs when there is an extra step between the reservoir and the host. An example of this would be picking up a cold by touching the tissue that your fellow patient had sneezed on, rather than being sneezed on personally. Similarly, some microbes (like anthrax, which we’ll learn about later) can hang around in the environment for extended periods – picking them up from wood or other natural sources is another example of indirect transmission.
Both direct and indirect transmission are called horizontal forms of transmission, which is different than vertical transmission. Vertical transmission involves pathogens that are passed between generations – the best example of this is mother-to-child transmission of the HIV virus through maternal blood.
Admittedly, this idea of a “generation” might seem tricky when trying to distinguish vertical from horizontal transmission. If an adult passes a disease to a child by sneezing on them,
is this direct or vertical transmission? It’s direct transmission – the reservoir and the host may be in different generations, but the transmission of the microbe is not a result of this generation gap. In other words, we only use the term vertical transmission when the pathogen is spread through things related to birth, like sperm cells, eggs, maternal blood in the womb, or during birth.
4. Lots of Fences for Bad Neighbors
With all these ways that microbes can get inside our bodies, what tools do we have to we have to stay healthy? Many are probably already familiar to you, such as drugs and vaccines, but others you may not think about as much.
What is the difference between a drug and a vaccine? A drug is, generally, a particular chemical compound that kills a microbe. It could cause a bacterial cell wall to rupture, or prevent a virus from replicating. Either way, it is an active response to an infection, after the microbe has already invaded the body.
In contrast, a vaccine is a pre-emptive strike, an attempt to prevent the microbe from invading in the first place. Vaccines are effectively early warnings: a doctor injects a patient with a particle that is somehow related to the microbe. It could be a molecule contained inside (like a distinctive piece of RNA or protein), or small amounts of the microbe itself. The body has an army of white blood cells specially trained to recognize messages like this, but their ability to respond is limited by what “warnings” they’ve received. Vaccines prevent infection because
these white blood cells receive the message and generate chemical defenses specially suited to fighting off a particular pathogen. Thus, when they encounter this microbe, they are already prepared to destroy it before it infects the body.
There is also a third class of treatments that may not be as obvious, called control measures. These can be public health projects like chemical spraying to get rid of mosquitoes, or a screen on your own porch to keep out these pests and the disease-causing microbes that live in their saliva. The important point is that control measures try to prevent the microbe from spreading in the environment, so we have less chance of coming into contact with it and getting sick.
We hope this introduction has been informative, and, with these few basic points in mind,you enjoy your further explorations into the sometimes confusing, often terrifying, but always fascinating world of infectious diseases. Enjoy!