A second laboratory worker has been infected with the AIDS virus, officials at the National Institutes of Health announced. They said the infection may have resulted when the worker's hand was accidentally cut during the process of preparing concentrated virus. The infection was first detected in May, 1986, when a blood serum sample obtained about seven months after the accident gave a weak positive indication of infection with the AIDS virus, health officials said. But they said the worker only recently learned of the infection because of a breakdown in communication. The first laboratory worker to contract the virus on the job was infected as the virus was being prepared in a concentrated form in a centrifuge, officials said. They believe the equipment leaked.