There are many causes that can cause unwanted, significant weight loss. Important causes are: cancer, COPD, heart failure, stomach and intestinal complaints and some chronic diseases. Weight loss can also be a side effect of the use of certain medicine, drugs or alcohol. An HIV infection can also cause weight loss.
Sometimes no good explanation for the weight loss can be found. Then the weight loss is called 'unexplained'.
We only speak of weight loss if you have lost 5 percent of your original weight in six months, without being actively involved (i.e. not dieting or exercising extra).
Example: losing 5% means that someone who weighs 80 kilograms and loses 4 kilograms in six months.