The term “lesion” is a non-specific one. From Wikipedia you can see that it is “any abnormality in the tissue of an organism; usually caused by disease or trauma,” and that is essentially correct.
Scanning the brains of people without illness randomly, particularly in old age, is bound to turn up lesions in many of them, as abnormalities of the brain increase with age.
It is important not to confuse these “lesions” that occur in people who are otherwise well with the “lesions” that occur in MS. Because MRI images are just a pictorial representation of the anatomy of various tissues, and while they’re more sensitive than CT scan images, these MRI images are not particularly sensitive.
Brain lesions look much the same on MRI no matter what causes them. The Rotterdam Scan Study found about 95% of older people who are randomly scanned have lesions in their brains. These lesions mostly represented the small areas of brain that die as blood vessels age and fail over time. In MS however, these lesions usually represent areas of demyelination, and the lesions are usually distributed in the brain in a very typical pattern, enabling radiologists to say that the lesions are typical of demyelinating disease.
People with MS often wonder why radiologists don't just say the patient has MS, but it is not that easy, because lesions are non-specific. So, in a person with symptoms suggestive of MS, and lesions in the typical MS pattern on MRI, a clinician may say the diagnosis is likely to be MS.
But that is why, the neurologists often do other tests and wait until a second episode with new MRI lesions before they make a definitive diagnosis.
In the OMS book, on page 61, you’ll read about the Italian imaging study that found 10% of relatives of people with familial MS (families with a history of MS), 4% of relatives of people with sporadic MS (no other MS in the family), and 0% of the general population had lesions suggestive of MS on MRI scanning. This is more relevant to people with MS than the Rotterdam Scan Study is.