''These people come to the United States and bring their disease with them,'' said Dr. Thomas Rea, a dermatologist at the University of Southern California who treats Carlos and 400 other leprosy patients at County-U.S.C. Medical Center. A Little-Understood Disease

Leprosy, also named Hansen's disease, for the Norwegian doctor who discovered leprosy bacteria in 1873, is a little-understood disease that over a period of many years attacks body tissue, especially skin and nerves. If untreated, it can lead to spots or sores on the body, nerve pain, accidental hand and foot mutilation stemming from victims' inability to feel pain through damaged nerves, and even death from complications.

In the first nine and a half months of this year, 193 cases of leprosy were newly reported in the United States, as against 160 in the same period of 1982, said Dr. Charles Shepard, chief of the leprosy section at the Federal Centers for Disease Control in Atlanta.

With the exception of a few years, the number of leprosy cases reported in the United States has increased annually, more than doubling from 103 in 1963 to 231 in 1982, he said. There are 4,000 to 5,000 leprosy patients in America and 11 million worldwide, most in India and Southeast Asia.

''The increased incidence is due entirely to imported cases,'' said Dr. Robert Jacobson, clinical chief for the United States Public Health Service's leprosy facility in Carville, La.