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Explain xkcd: It's 'cause you're dumb.
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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."

Explanation

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 conditions, 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. Statistics show that flu vaccine recipients are also less likely to die from a variety of other causes, but this is believed to be either because someone with the flu is more likely to have a heart attack, car accident, etc., or because of the healthy user effect. Even if there is a slight protective effect, it will certainly not completely 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, and one severe sunburn can also trigger it. As ultraviolet radiation is a form of electromagnetic energy, it cannot be prevented by vaccination, but use of sunscreen with a high SPF factor can provide protection for a few hours. Melanin provides some natural protection, so skin cancer occurs disproportionately in some races, although it can occur in any race. Stimulating melanin production through controlled exposure to UV radiation for cosmetic purposes is called "sun tanning", but physicians now recommend against the practice, because the UV radiation used for tanning can also cause skin cancer.
  • 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. Antivenom is more effective the sooner it is administered; for venomous snakes in North America, it is generally recommended to be treated within six hours of being envenomated.
  • Raw 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 impacted by a disaster or the pipes carrying the treated water experience a failure/break, the government may advise residents to boil their water before drinking it. This is likewise advised for people living or travelling 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. There was a brief fad in 2017-2018 of selling bottled raw water in health food shops, advertised as a "probiotic". While it is true that untreated, unfiltered spring water has more microbes in it than purified water, these microbes are not beneficial to human life and may even kill you in some cases.
  • Computer viruses are computer programs that now usually spread through networks via infected devices, attachments, and websites (early computer viruses were often spread by floppy diskettes). 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.
    • Megan specifically mentions clicking on links that have "weird Unicode in them"; this may be referring to an IDN homograph attack, in which attackers register domain names that use Unicode characters that resemble ASCII characters to trick users into thinking they are visiting a website belonging to a trusted party. For example, an attacker could register a website with the URL "xкcd.com", in which the Latin letter "k" is replaced by the Cyrillic letter ka (к), and then send emails to trick users into visiting that site and attempting to log in. The attacker can then attempt to use the supplied passwords on more important websites, as in 792: Password Reuse.

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.

Transcript

Ambox notice.png This transcript is incomplete. Please help editing it! Thanks.
[Megan walking into frame from the left, with Cueball outside of panel to the right.]
Megan: Yesss, I got my flu shot.
Cueball (off screen): Nice! I got mine a few weeks ago.
Cueball (off screen): Immunity buddies!
[Megan spreading her arms wide, with Cueball now in-panel to the right of Megan.]
Megan: 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
[Frameless panel with Megan flexing her arms and Cueball in panel again.]
Megan: 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 to the left away from Cueball.]
Megan: 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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