Molluscum contagiosum is a common viral infection among children that causes raised pearl-like nodules or papules to develop on the skin, usually on the face, neck, armpit, arms, and hands. Typically the skin eruptions occur in lines where the person has scratched (scratching or other irritation causes the virus to spread in a line or in groups). It can be spread to other parts of the body or to other people by direct contact with the lesion or by touching objects that have the virus on them. The papules are painless, and there is usually no inflammation. It may be a few weeks or even up to six months after contact with the virus before symptoms develop. (Among adults, molluscum contagiosum often appears on the genitals and is considered a sexually transmitted disease.)
Unfortunately, the infection may persist for some time - for months as in your son's case or even years - but the lesions usually do go away by themselves without leaving scars. Since the condition is self-limited and doesn't cause pain, no treatment is necessary, although medications may be used to remove visible or embarrassing legions or to prevent them from spreading to other areas of the body or to other people.
It is unlikely that molluscum is directly related to the other medical problems you son has had. However, the disease can be worse than usual among people who have other conditions that affect the immune system. You might try boosting your son's immunity with astragalus (Astragalus membranaceous), a Chinese tonic herb. Astragalus is a safe and effective means of increasing resistance to disease and fighting off chronic or recurrent infections. Buy it in capsule form in a health food store and give your son half the adult dose for at least two months.
Another possibility would be to consult a practitioner of homeopathic medicine, which often works well in children.
Andrew Weil, M.D.
More information on croup and scarlet fever.