My wife has been worrying about cancer in the past from time to time. Five years ago, she told me there was a hard spot around her neck, and started worrying about all kinds of sickness she could search from Google. We spent lots of time and money, and it turns out nothing there. Since then, she doubted many times on some parts of body and always related them to cancer, which were later proved nothing again and again. Lately, she is worrying about her back, rub it frequently, so that her skin on back was peeled off. First time, doctor told her there was nothing. Second time, she went to a different doctor and he gave her some antibio medicine for slight infection. No matter what doctor or I say to comfort her, she always thinks people are ignoring her serious concern. But I am pretty sure she is alright but may not be mentally. How can I help her?
She has something called hypochondria which is the same disease I suffer from. Hypochondria means excessive health anxiety. Let me tell you some key points. If she is worried too much she'll start producing REAL SYMPTOMS in her body. I was worried about a spot in my mouth for 2 years and had misdiagnosed it as leukoplakia using google. As a result I developed chronic neck pain. Never let her do that. Google often provides links to websites with INCOMPLETE information about a disease intended for the general public and NOT proper medical websites. A lump in the throat is actually normal as lymph nodes are present there. If it was cancer indeed, 5 years would have been ample time for the tumor to grow in size and cause fever and all other cancer symptoms. Health anxiety is not something you can completely get rid off. It's only a moment of time before you discover some new virus over the internet that spreads through touching/sneezing and start obsessing about it. You need to tell her to relax and make her believe that the human body is more complex and well suited to fight infection and diseases than she knows. Just google the placebo (also nocebo) effect and tell her read about it. It proves how powerful can human psychology be when it comes to physical well-being. To a hypochondriac repeated reassurance is required because they tend to fall into a cycle of constant worrying and re-analysing the cause of their worry. The key to overcome this is to break this cycle. Practicing meditation on a daily basis helps calm the mind and allows it to think more clearly. Also meditation helps you to analyse yourself in a quieter way. That way you learn to control your anxieties. I will strongly recommend Michael Sealey's YouTube channel for video-guides to meditation and hypnosis. Begin with this one. These definitely helped me more than anti-anxiety medication ever did.
You can't cure anxiety but you can definitely learn to ignore it.
A psychologist could help her, e.g. using Cognitive behavioral therapy. It's likely that whenever she feels that something doesn't feel quite right and the idea that this could be cancer crosses her mind, she doesn't have enough knowledge to dismiss such thoughts on their merit. If she is taught about this, then she can start to suppress such thoughts.
But it will then still take some time before these negative thoughts will stop to cross her mind. This is because such thoughts originate at the subconscious level, the brain parts involved can flag that something seems to be wrong even if you at the conscious level know that there is no problem and that this is false alarm. The alarm will then still be there but she can ignore it, as long as she knows that it is indeed false alarm. The reason why this is going on is because by believing in what she felt she amplified the signals which have had the effect of programming certain brain parts to act this way. These brain parts don't know that this is a bad thing. If she had been learning a new skill then the same sort of processes would have led her to become better at performing that skill.
Since the wrong programing is still present, this means that even when she knows that simply feeling something somewhere in her body does not mean she has cancer, the signals she perceives will still be amplified and felt in an exaggerated way. She needs to appreciate this effect in order to continue to be able to dismiss the idea that she may have cancer. But lowering stress levels simply by not believing that she has cancer and also relaxation methods as mentioned in Sona's answer will already go some way to reduce whatever abnormality she is perceiving.
Tell her to breath deep every day for atleast 5 to 10 minutes she can start by doing for 2 minutes it will calm her down and she would worry less as her mind would relax . You can make a comfortable and loving environment for her so that she always know that you are there whatever happens . These easy things can be done at home . It would be hard to be patient with her but when she would see you are there every time it happens to console her , may be she realise its all her imagination . Or you can go with Count Iblis's answer and contact a psychologist .
As other people have indicated, what you're describing is definitely a mental illness. In the DSM-IV this was called hypochondriasis and is also sometimes called health anxiety.
Technically it's now split into two separate conditions as of the adoption of the DSM-V but it's still worthwhile to look at the diagnostic criteria from the DSM-IV:
A. Preoccupation with fears of having, or the idea that one has, a serious disease based on the person's misinterpretation of bodily symptoms.
B. The preoccupation persists despite appropriate medical evaluation and reassurance.
C. The belief in Criterion A is not of delusional intensity (as in Delusional Disorder, Somatic Type) and is not restricted to a circumscribed concern about appearance (as in Body Dysmorphic Disorder).
D. The preoccupation causes clinically significant distress or impairment in social, occupational, or other important areas of functioning.
E. The duration of the disturbance is at least 6 months.
F. The preoccupation is not better accounted for by Generalized Anxiety Disorder, Obsessive-Compulsive Disorder, Panic Disorder, a Major Depressive Episode, Separation Anxiety, or another Somatoform Disorder.
As others have indicated, CBT and possibly medication are effective treatment for this.
You've probably gathered as much but merely reassuring her really isn't an effective solution.
As a disclaimer, I don't personally suffer from this particular condition (so someone correct me if I'm off base here), but based on my experience and reading on other anxiety disorders it's also very important to find a way to interrupt the associated behaviors (e.g. unnecessary medical visits, receiving unnecessary treatments, etc.) as those just serve to perpetuate the anxiety. (This is analogous to obsessive-compulsive disorder; compulsive hand-washing may reduce your anxiety in the short term but in the long term it keeps you trapped in the disorder). This is definitely something that she can work with a therapist on though.