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Explain xkcd: It's 'cause you're dumb.
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Revision as of 12:19, 18 December 2013

Welcome to the explain xkcd wiki!
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Flu Shot
"Wait, how often are you getting bitten by snakes? And why are you boiling water?" "Dunno, the CDC people keep showing up with complicated questions about the 'history of the property' and 'possible curses' but I kinda tune them out. At least one of them offered me the flu shot."
Title text: "Wait, how often are you getting bitten by snakes? And why are you boiling water?" "Dunno, the CDC people keep showing up with complicated questions about the 'history of the property' and 'possible curses' but I kinda tune them out. At least one of them offered me the flu shot."


Ambox notice.png This explanation may be incomplete or incorrect: Created by a FLU VIRUS. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon.

In this comic, Megan tells Cueball that she got a flu shot, which is a vaccine to prevent getting the common flu. She then goes on to claim she doesn't have to worry about being bitten by bats, but the worry with being bitten by bats is rabies, not the flu. This implies she got the two confused. Then, she goes on to claim to now be immune to other abnormal stuff, such as poison ivy, snake venom, contaminated water, and computer viruses.

The flu shot consists of inactivated viruses from three different strains of the flu, which are those judged by the World Health Organization (WHO) to most likely be in wide circulation in the following flu season. Because the influenza virus comes in many strains and mutates rapidly, the flu shot is generally less than 60% effective at preventing flu infections; this is a positive effect for health outcomes, but it's not exactly what most people think of as "immunity", especially compared to e.g. the 97% effectiveness of the MMR vaccine against measles and rubella, and it will certainly not prevent harm from coming to Megan by the other sources of infection or poison she mentions (except to the extent that all of these things will be even worse for her if she is also sick with the flu):

  • Rabies is a viral disease that causes brain inflammation, which in turn causes symptoms including aggression, fear of water, and violent uncontrollable limb movements. It can be carried by almost any vertebrate animal, but bats, raccoons, and wild dogs are the stereotypical carriers. There is a rabies vaccine, but it is generally only administered to pets and humans who work extensively with animals.
  • Poison ivy is a vine which produces an oil, urushiol, which chemically reacts with membrane proteins on the skin cells it contacts, which in turn tricks the immune system into attacking those cells. Some people are not affected by poison ivy, but as it is an allergic reaction, people often become more sensitive to poison ivy upon repeated exposure. There is no known vaccine or other permanent preventative treatment against urushiol sensitivity, although there are several creams that can be applied in advance of expected poison ivy exposure to reduce the risk of contacting the chemical.
  • Sunburn is caused by exposing the skin to high levels of ultraviolet radiation, such as by playing outside in noontime sun for an hour or two without clothing or sunscreen. Repeatedly getting sunburned can increase the risk of skin cancer later in life. As ultraviolet radiation is a form of electromagnetic energy, it cannot be vaccinated against, although some humans have greater tolerance for UV radiation due to melanin production in their skin. Stimulating melanin production through controlled exposure to UV radiation is called "sun tanning", although this is generally done for cosmetic rather than medical purposes, and the effect will fade over time if UV doses are reduced.
  • Snake venom is not one single compound, but several proteins and molecules produced by venomous snakes to inject into prey. Different snakes' venoms have different effects, so there is no single vaccine or antivenom for all snake bites, but antivenoms are produced by a process similar to vaccination. Small doses of venom are injected into host animals, such as horses, to provoke an immune response; the resulting antibodies are then stored to be injected into snakebite victims, where they will bind up and inactivate the toxic proteins and mark them for disposal by the immune system.
  • Untreated water may be contaminated by bacteria, protozoa, parasites, and chemical pollutants. Boiling water will kill off any biological contaminants, which will prevent food-borne diseases such as cholera, dysentery, and giardiasis. In developed areas of the world (which presumably most of the XKCD characters live in), boiling water is generally not necessary due to municipal water treatment, but if those treatment facilities are interrupted by a disaster, the government may advise residents to boil their water before drinking it. This is likewise advised for people living or traveling in less-developed areas, like backpackers or farmers. Some of these diseases can be prevented by vaccines, but because there are so many microscopic life-forms in water, it is not possible to vaccinate against all of them.
  • Computer viruses are computer programs that spread through networks via infected devices, attachments, and websites. They can cause harm directly by taking up computer cycles and network bandwidth, but nowadays they often perform other tasks for their creators, such as exfiltrating financial information or encrypting files and demanding ransom for the keys. Computer viruses can be recognized and blocked or deleted by software that scans incoming files and links against known computer virus patterns, which is analogous to vaccination, but there is no vaccine that can be administered to Megan which would protect her computer.

In the title text, Cueball asks Megan why she is subject to all of the above maladies. She responds that some members of the U.S. Centers for Disease Control (CDC) have evidently speculated that Megan is subject to all of the above maladies due to a curse on her property. In many works of fiction, building over sacred or desecrated ground (such as the stereotypical "ancient Indian burial ground", mentioned by Megan back in 782: Desecration) results in curses befalling the builders and/or occupants of such properties, although usually those curses express themselves in more fantastical ways, such as disruptive poltergeists or re-animating the dead.


Ambox notice.png This transcript is incomplete. Please help editing it! Thanks.
Megan (walking into frame from the left): Yesss, I got my flu shot.
Cueball (off panel): Nice! I got mine a few weeks ago.
Cueball: Immunity buddies!
Megan (spreading her arms wide): Now I can finally get bitten by all the bats I want!
Cueball: No, that's rabies, that's not what—
(closeup of Megan's head)
Megan: I'll be able to roll and play in the poison ivy without a care in the world!
Cueball (off screen): Why would you do that even if the shot did
Megan (flexing her arms): No more slathering on sunscreen. No more rushing for antivenom after a snakebite. And now I can stop wasting time boiling contaminated water before drinking it!
Megan (running off toward the left): Gonna click on every URL in every email I get, even the ones with IP addresses and weird Unicode in them!
Cueball: You know what, sure, go for it.

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