That stops reproduction of the virogen, and tags it so antibodies can form.

It is thought to occur when weakly cross-reactive antibodies form bridging complexes to facilitate uptake and replication of related but non-identical variants.

Immune hemolytic anemia occurs when antibodies form against the body's own red blood cells.

"And lo and behold, it circulated forever and no antibodies formed," Dr. Davis said.

Idiopathic autoimmune hemolytic anemia is an acquired disease that occurs when antibodies form against a person's own red blood cells.

For people who have that genetic makeup, he said, "if some environmental insult occurs, it seems to cause antibodies to form to tissues in the body."

The scientists do not know how long the men have carried the virus; antibodies usually form within about six months of infection.

The most common current test for the disease looks for antibodies to the infection, but the antibodies do not form until four to eight weeks after a person is infected.

Now, however, in optically based biosensors, antibodies can form an integral part of the sensing device.

It becomes positive only after antibodies to H.I.V. have formed, a period that can range from a few weeks to months.