The study followed nearly 7,000 people age 55 and older for an average of seven years. Over that time, 706 of the participants developed dementia. People who were current smokers at the time of the study were 50 percent more likely to develop dementia than people who had never smoked or past smokers.

Smoking could affect the risk of dementia through several mechanisms, according to study author Monique Breteler, MD, PhD, of Erasmus Medical Center in Rotterdam, the Netherlands, and member of the American Academy of Neurology.

Smoking increases the risk of cerebrovascular disease, which is also tied to dementia, Breteler said. Another mechanism could be through oxidative stress, which can damage cells in the blood vessels and lead to hardening of the arteries. Smokers experience greater oxidative stress than nonsmokers, and increased oxidative stress is also seen in Alzheimer's disease.

Oxidative stress occurs when the body has too many free radicals, which are waste products produced by chemical reactions in the body.

Antioxidants in the diet can eliminate free radicals, and studies have shown that smokers have fewer antioxidants in their diets than nonsmokers, Breteler said.

The researchers also looked into how smoking affects the risk of developing Alzheimer's disease for people who have the gene that increases the risk of Alzheimer's, called apolipoprotein E4, or APOE 4. They found that smoking did not increase the risk of Alzheimer's for those with the APOE 4 gene. But for those without the APOE 4 gene, smoking increased the risk of Alzheimer's. Current smokers without the Alzheimer's gene were nearly 70 percent more likely to develop Alzheimer's than nonsmokers or past smokers without the Alzheimer's gene.

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The team say people who have mental health problems are more likely to develop other health problems and may be susceptible to alcohol abuse, smoking and poor diet and physical fitness.

All these in turn lead to an increased risk of heart disease, cancer, strokes or diabetes.

When they do develop a serious physical illness, they are also less likely to get the medical help, social support and treatment they need.

Families suffer too and the team say evidence exists from India and Pakistan, that mothers who are depressed are more likely to have a malnourished child.

While the choice between bed-nets or anti-depressants is one no-one wants to make the health professionals say the real solution is to increase the amount of money being given to poorer countries for health.

The researchers said that unless governments adopted wide-reaching strategies, poor countries will be the worst affected in terms of mental illness in the near future.

The experts say basic mental health services can be provided cheaply and simply, especially if they are made part of general healthcare.