WASHINGTON — The virus that causes AIDS can easily infect individuals through the mouth, even when they have no oral sores or bleeding gums, a series of studies in monkeys suggests.

　The findings provide the first strong evidence that receptive oral sex puts people at risk of acquiring the fatal disease. 　Researchers and public health officials stressed that there was still no evidence that casual contact, or even mouth-to-mouth contact such as kissing or sharing utensils, can spread HIV, which causes AIDS. 　But they said the monkey studies offer evidence that an oral dose of semen from an HIV-infected man can cause AIDS in another person if it comes into contact with tissues in that person’s mouth.