Medicine: AIDS Research Spurs New Interest in Some Ancient Enemies

The invader is tiny, about one sixteen-thousandth the size of the head of a pin. It consists basically of a double-layered shell or envelope full of proteins, surrounding a bit of ribonucleic acid (RNA), the single-stranded genetic molecule, and often enters the bloodstream of its victim after sexual contact. It is an AIDS virus, and its intrusion does not go unnoticed. Scouts of the body's immune system, large cells called macrophages, sense the presence of the diminutive foreigner and promptly alert the immune system. It begins to mobilize an array of cells that, among other things, produce antibodies to deal with...