Cyberchondria
Hypochondria refers to an obsession-level certainty held by some people (hypochondriacs) that they have a serious medical condition, despite not having such a condition.
This term has been colloquialized and blurred to sometimes refer to someone who talks a lot about having a perceived ailment, or folks who always seem to be sick, but the International Classification of Diseases has categorized hypochondria as a mental and behavioral disorder, and the Diagnostic and Statistical Manual of Mental Disorders has categorized is as a somatic symptom disorder, which means it's the result of a maladaptive and/or excessive fixation, with more extreme versions typically labeled illness anxiety disorder.
The practical consequence of this disorder is that the afflicted person will feel certain they have some kind of disease or other health issue, and no matter what medical professionals say or do—no matter how many tests and labs are done—no amount of evidence is enough to convince the patient they're okay (and that this perception is not realistic).
To the person suffering, though, the experience will tend to feel quite real, and the sense of wrongness can go on to pervade every aspect of their lives, making this similar in some ways to other types of obsessive compulsive disorders.
Cyberchondria is a newer term that overlaps with illness anxiety disorder but which focuses on the tendency of patients to become fixated on searching the internet for medical information, that searching making them feel ever more certain they've got something they don't, or that something they do have is something else (often, but not always, something more serious).
This term is not a formal medical label, and it sprawls to encompass many different concerns.
Key among said concerns is that because of how health-related information is disseminated online, and because of how low-quality a lot of that information is, health-googling can enflame anxiety in patients, and because it's possible to quickly accumulate what might superficially seem like evidence for any prior belief or intuitive suspicion (that evidence tending to be anecdotal or otherwise subjective) patients will sometimes develop a certainty about their suspicions that becomes difficult to dislodge, even when challenged by facts and the fruits of medical experience.
In other words: people have gut-feelings about things that can be reinforced by other people's gut-feelings, discovered online, and the huge collection of bad/wrong/ignorant information available to most of us all day, every day can make us immune to (or cause us to discount) more legitimate, data-backed information.
This, in turn, can amplify anxieties we latently feel about whatever it is that’s wrong with us (or suspect might be wrong with us).
Secondarily, this can balloon existing concerns and suspicions about the medical community (and maybe even scientific inquiry, more broadly), which can then make us even more reliant on web-shared anecdata and (at times well-meaning) charismatic snake oil salespeople.
This term also gestures at a controversy and ongoing challenge within scientific fields related to how they communicate their findings to non-specialists.
A lot of the bad medical information and advice available on the internet is so appealing because it's more accessible than the more legit stuff, and the subjective, anecdotal info is communicated in a way that's more resonant (and thus, more trustworthy-seeming).
Part of the responsibility here is on patients, as we could all be a little more responsible for our own consumption (and then hopefully a little less credulous to mis- and disinformation, in general).
But part of this issue originates with the medical world, which could do with a rethink of how it communicates findings and builds trust.
Some research has shown (and intuition supports this finding) that when doctors respectfully listen to and consider the concerns and experiences of patients, even to the point of engaging with their personal research (what they discovered online), sharing objective information and data (better research sources, basically) with folks who are going to be googling for information anyway, that can empower patients to become better advocates for themselves, helping them find and understand more legit info while also increasing the perceived credibility of those doctors (who are then more capable of reassuring their patients, staving-off potential cyberchondria-anxiety spirals).
Rather than demanding people trust in their authority absolutely, then, the medical community might be better served by arming motivated patients to more capably keep tabs on their health, over time inoculating them against bad information rather pushing them toward it.