Is it possible for a contagious disease to cause the symptoms of a recognized mental illness? Is it possible for a specific mental illness to be appreciably contagious beyond a small group of people, or beyond a group of people who already have mental problems?
It seems that a lot of illnesses that were once thought to be purely psychic have an identifiable physical component. Is it possible for any of those physical components to be transferrable from person to person?
Sure. Syphilis, left untreated, can manifest as a dementia, which used to be called “General Paralysis of the Insane”, or GPI, until it was recognised as tertiary syphilis.
Rabies similarly has symptoms which would be regarded as psychotic.
There may be many other such diseases.
Whan a diagnosis of mental illness is made, an organic cause should always be ruled out first.
There are several diseases that have effects on mental health, but I would think most of them would be highly unlikely to spread to large numbers of people.
Any disease that causes high fevers can cause delirium, which could look like dementia.
Kuru and CJD can cause symptoms that could look like a psychosis or dementia, but you have to be infected by prions.
Poisoning with Ergot caused St Vitus’ dance, but again, you have to eat the Ergot infected grain products and physicians would be unlikely to consider the symptoms suggestive of mental illness nowadays.
Ingestion of hallucinogenic substances, adulterated alcohol etc could cause hallucinations and delusions, but would again be limited to those who had taken the substances.
Dementias, temporal lobe epilepsy and some brain tumours can cause hallucinations, altered behaviours and other symptoms the could be wrongly identified as a mental illness, but those aren’t contagious.
You might want to think about mass hysteria, “folie a deux” and such things as contagious mental illness, but I’m not sure that experts in the field would.
Many mental illnesses such as schizophrenia, Bi-polar disorder and depression have a genetic component, but I’m not sure if that is what you’re getting at.
Untreated strep throat can lead to obsessive-compulsive disorder in a small number of children. Cite.
I knew about CJD, syphilis, rabies, et al. but I apparently blanked on all of them, because they weren’t what I was thinking of. That is, I’m not really interested in simple degerations of the mind: Those diseases have interesting symptoms, but the final result is the complete degeneration of the brain. I want to know if something like clinical depression or schizophrenia is contagious in the same way syphilis would be: Stranger-to-stranger transmission.
Ergotism, bad booze, and LSD consumption for that matter aren’t really contagious, so they aren’t part of this thread at all.
“Folie a deux” is closer, but as I understand it, it’s very limited: The two people must be emotionally linked somehow, or there is no way they could share in the same delusion.
Strep causing OCD is precisely what I was thinking about, because I read about it in Reader’s Digest a couple years ago and it has festered in my mind like a herpes virus in remission. (No, I don’t subscribe. I read the article in a doctor’s office, which was either ironic or really great marketing.)
Encephalitis lethargica comes to mind, incidentally, but it is just on the border of what I’m talking about. I suppose I’m falling into a definitional trap, but I still think there’s some difference between a neurological disorder and a mental disorder. The people who had the “sleepy sickness” didn’t become delusional or otherwise turn into different people, they simply lost their link to the outside world.
I’m willing to admit that my topic could be malformed. But the interest the OCD-causing strep holds for me is the subtlety of the causative chain, and the profound change it effects: The immune system, kicked into action by the strep bacteria, begins to kill off the basal ganglia in an autoimmune response. The fact that something as personality-altering as OCD arises from that is amazing, as well as the fact that it doesn’t completely destroy the brain in the process. Is there anything else like it that we know of?
The West Nile virus can cause encephalitus, which can cause neurological problems that are similar to mental illnesses in some ways.
Researchers also think that schizophrenia may be linked to an endogenous retrovirus called HERV-W carried on human RNA, found in the cerebrospinal fluid of 8% of chronic schizophenics and 30% of acute scizophrenics. As I understand it, it’s not exactly a contagious virus, but is instead an ancient virus that worked it’s way into the genetic code millions of years ago and may now cause schizophrenia when triggered by something else, like the herpes virus or certain diseases in early childhood or in utero.
There’s some evidence that borna virus may cause depression in humans
I’ve not kept up with this, so have no idea if the connection still stands.
There are some genetic defects that cause problems frequently mis-diagnosed as mental illnesses, but which are really a physical problem.
For example, some people have an inability of their body to process & remove heavy metals (like iron & copper). Eventually enough of the metal builds up in their body that it starts to cause symptoms. Typically these problems start to appear in the late teens or early 20’s, which is a common stressful time where mental problems can start, too. So they are often diagnosed at first as mental illness. They may be treated as such for some time (unsuccessfully) before the physical problem is recognized. Though this happens less often now, since blood tests, etc. which will identify the real problem are more commonly used.
And of course, the genetic defect causing this often runs in families. Sometimes this causes the appearance of contagious mental illness. “Susie went crazy about age 19; now her younger brother is starting to go crazy too.” But in fact, he is approaching the same age, and his body is also starting to suffer from the build-up of metals in it. So this might look like “contagious insanity”, though it isn’t really.
P.S. I don’t know if there really is “contagious insanity”, but there can certainly be “carriers” for that. I’ve known several people who manage to drive everyone around them crazy! 
This is a fascinating topic, and I’m really happy that it’s apparently very rare for the symptoms of a mental illness to be caused by a contagious pathogen. The nightmare scenario of a disease getting loose that caused, say, mass schizophrenia is unlikely in the extreme.
A virus that causes depression is truly frightening.
what about this beastie. I mean you would have to say the rats went mad!
from http://www.guardian.co.uk/life/thisweek/story/0,12977,1048642,00.html
"A relative of the malaria bug, the parasite is an intracellular protozoa called Toxoplasma gondii and its possible risks to unborn children have been known for some time - hence pregnant women are urged not to clean the cat litter tray, for example.
More recently, scientists have begun to unpick its effects on behaviour, discovering that infected rats lost their aversion to cats and so were more likely to be eaten, thus allowing the parasite to complete its life cycle. There are fewer experiments in humans, but results from studies of students and conscript soldiers in the Czech Republic in the mid-1990s highlighted the fact that infected people showed different personality traits to non-infected people - and that the differences depended on sex. Infected men were more likely to be aggressive, jealous and suspicious, while women became more outgoing and showed signs of higher intelligence."
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