so what are they?
I could give the clinical answer, but, instead, I'll tell you what I think. I think society determines what it considers 'normal' and anything that falls outside of that range gets labeled. Sometimes that label is warranted, in that the person with a 'mental illness' really cannot function well for themselves or in society and needs some sort of assistance in coping/improving so that they can function. I, personally, would put more emphasis on being able to function well for themselves, and then in society, unless the person really was a danger to themselves or others in some way. I also think we have very little idea of what 'normal' is or how it's achieved. I think the societal push for 'normal', depending on what that 'normal' has been defined as, is sometimes detrimental to mental health overall. Also, I think some mental illnesses are normal mental/physical reactions to abnormal stressors.
what do you think causes them?
Chemical imbalances in the brain, or brain injury.
are you scared of people who are mentally ill?
Not in general, but I would be if they were violent. I've only known one person with a mental illness who was violent enough to scare me, but he seems to have that under control.
do you know anyone who is?
My oldest brother hasn't been diagnosed, but we're all pretty sure he has OCD. I know a few other people with OCD, also. If we count brain injury, then my mom--her stroke caused vascular dementia. I know several people with diagnosed or undiagnosed anxiety disorder, depression, or panic disorder. My sister-in-law used to have an eating disorder. I know a couple of people with sex addictions, and one pathological liar. I would guess that some of the people I know fall somewhere on the autism spectrum, though I don't know if any of them were diagnosed. I know a couple of hoarders. One of my grand-nephews might be ADHD.
what do you think impact on society is?
I'm really only familiar with US culture, and only small sections of that, so my answer is going to reflect that. I think a lot of professions and corporations related to the mental health industry see profit in promoting mental illness. I think that we like quick fixes here in the US and dislike anything that seems to express weakness, but we also want to see ourselves as compassionate. I think the result is dehumanizing to society as a whole because we tend to see mental illness as something to be fixed and fixed quickly so that we can show how compassionate we are while expending as little effort as possible in making sure that everyone fits a certain standard of 'normal'.
what do you think is the most effective way of treating them?
It depends on the severity and type of mental illness. Mental illness due to brain injury might not be treatable, depending on the injury. I think that several factors have to be taken into consideration when determining the best treatment. I think we have a tendency to over-medicate here in the US. While some mental illnesses do need to be treated with medication, others might improve or resolve with other forms of treatment (therapies, diet, changes to the environment, etc.).
do you think you can tell who does or doesn't have a mental illness just by a sole interaction?
For the most part, no. Even if you can tell that something seems off from a sole interaction, you might attribute it to something other than mental illness (quirkiness, substance abuse, having an off day, etc.).