A MUCH-DELAYED ground-breaking course to help China combat drug abuse and cut HIV infection starts today at the Chinese University. The 20-day course, sponsored by the World Health Organisation and the United Nations Drug Control Programme, is the first of its kind, according to convenor Professor Chen Char-nie of the Department of Psychiatry at the university. The 24 delegates - 18 from China, two from Macau and four from the territory - would spend most of the time visiting drug centres such as Hei Ling Chau to gain practical experience in treatment and rehabilitation of abusers, said Professor Chen. ''It's not only lectures but practical experience of how to do it, so [the mainland delegates] can go back to China and teach others,'' he said. Hong Kong had the region's most ''complete and comprehensive'' programme for prevention and treatment, and had been quite successful at keeping HIV infection low among drug abusers, he said. In comparison, ''suddenly in the last five years China has been having a severe problem in drug abuse'' and there was little training in preventing the spread of HIV between abusers, he said. Hong Kong has only seven known drug abusers who have tested HIV positive, but the figure is likely to be a gross under-estimate because it relies on abusers agreeing to have a blood test. There are 450 known HIV cases in the territory and 17,500 drug addicts on the Government's registry last year. China had about 250,000 drug addicts at the end of 1992 and said in June that it planned to send addicts who refused treatment or those who relapsed to labour camps for up to three years. The course would aim to bring more humane methods while stressing prevention.