There are a lot of different types of medical quackery out there, and while some might be harmless, many tend to do more harm than good.
Here is a list of what is in my opinion to be the five most dangerous:
Five: Alternative Mental Health Treatments
While certain types of alternative mental health treatments might be useful, other types are not useful at all, and even harmful.
The particular ones that I'm most concerned with are exorcisms, conversion therapy, and vitamin therapy.
The reason why these therapies are so harmful is because it can keep a person from seeking real, legit mental health treatments. With exorcisms and vitamin therapy, it can cause a person's mental health problems to become worse due to the lack real mental health therapy, and it can leave a person reluctant to use actual legit therapies, or, like with conversion therapy, which is mainly used to attempt to "rid" a person of homosexuality, can actually cause mental illness, like depression.
Because these therapies simply don't work, and can even cause mental illness to get worse, it might cause a person commit suicide, or even cause a person to become violent.
Four: Spiritual Healing
This basically comes in two forms: Faith healing and energy healing.
Faith healing is basically the belief that God uses certain people to help heal the sick, and energy healing is the belief that a person can "channel" their energy to heal people.
Besides the fact that none of this has ever been proven to work, it's dangerous because it causes people to believe that they're being healed, or will be healed, by some outside energy force, and this belief causes people to forgo proven medical treatments, and instead, basically really do nothing and hope they will get better.
The reason why this is more dangerous then alternative mental health treatments is because you can usually get better if you start getting real mental health treatments, unlike with spiritual healing, where if you wait to long, whatever you have could kill you.
Three: Alternative Cancer Cures
Alternatives cancer cures are basically claims by some people claiming to be doctors, although usually they turn out to be con artists, that they can "cure" a person of cancer.
The reason why some people seek alternative cancer cures is because conventional cancer treatments can be pretty harsh, and because of this reason this is why some people seek alternative cures is because they don't want to go through harsher conventional treatments. The problem with these "cures" is that they just don't work, and sometimes even can make things worse, and while they may be less harsh then conventional cancer treatments, at least those treatments usually work.
The reason why this is more dangerous then spiritual healing is because sometimes it is better to do nothing then to do something that can cause more harm then good.
Two: AIDS Denial and Cures
Basically this is the belief that either HIV does not cause AIDS, but that conventional treatments causes AIDS, or that HIV can cured by simple means.
My problem with this is that not only are both completely false, it causes people who believe in this stuff to forgo proven medical treatments for HIV, and it also makes people not want to change their lifestyles which may have caused them to get HIV in the first place, so they end up spreading HIV to more people.
It should also be noted that several HIV positive AIDS denialists have died from AIDS related illnesses.
The reason why this is more dangerous then alternative cancer cures is because unlike cancer, HIV can be transmitted from person to person.
This is the belief that vaccines cause autism and other types of neurological disorders.
The problem with this belief is that every creditably study about this has shown that not only does vaccines not cause autism, or other neurological problem, getting vaccinated could actually prevent neurological disorders caused by certain diseases.
The reason why this is more dangerous then AIDS Denial, and in fact the most dangerous of all the different types if medical quackery is that not only is it so wide spread, but that it's harmful to other people, especially other children who's parents also didn't vaccinate their children, and infants who might be to young to receive certain vaccines, and the diseases that these vaccines are meant to prevent can kill a person allot faster then HIV, and is much more easily transmitted from person to person. This is why it is my opinion that anti-vaccine is the most dangerous form of medical quackery in the world.