Unsafe sex has, for the first time, become the main means of transmission of HIV/Aids in China, overtaking injecting drug use. The BBC's Jill McGivering looks at some of the issues this will raise.
In many ways, the pattern of spread of HIV/Aids in China was predictable.
HIV/Aids is increasingly affecting the mainstream population
It mirrors similar patterns in many other countries.
Initial clusters of cases in the 1980s and 1990s were attributed to specific causes.
Many infections came from contaminated blood transfusions, a product of poor screening and the then badly-regulated practice of buying and selling blood.
Most other cases were amongst injecting drug users and, until now, drug use was the main means of transmission nationally.
So news from Chinese officials that sex has now overtaken drug use as the main cause of HIV/Aids suggests confirmation of a new phase. It confirms that HIV is more fully entrenched in the mainstream population.
The news will also force the Chinese authorities to grasp a very painful nettle and pursue more aggressive mainstream education campaigns to prevent the further spread of HIV.
That is challenging for almost every society. For China, it will be particularly difficult.
Fear about a more rapid spread of HIV through sex comes just as China is starting to change its sexual behaviour.
In the recent past, it has been a conservative society - both in its attitudes and its practices.
That is changing. China's process of opening up to the outside world has exposed its population to more liberal ideas - from fashion to sex.
Greater freedom of movement has allowed millions of migrant workers to swap the watchful, generally repressive, eyes of their families and communities for the anonymity of the city.
The Chinese do not often talk openly about sexual issues
Male workers, away from their wives and parents, have more opportunity, at lower social risk, to expand their sexual horizons.
Some surveys, cited in the state media, suggest one in 10 sexually active men has bought sex from a prostitute. The real number may be higher.
Punitive official attitudes towards sex workers, who operate in a grey area legally, make it difficult to target them in education and health programmes.
Even if sex workers know about HIV/Aids, it can be difficult for them to insist that clients use condoms.
Premarital sex is also becoming more acceptable.
One recent survey of sexual attitudes found that more than half of the people questioned thought pre-marital sex was acceptable.
The percentage was highest amongst the young.
Embarrassment and horror
But when it comes to talking, sex is still a taboo subject.
Last year, I visited Ruili in Yunnan Province. The town, close to the border with Burma, is sometimes dubbed the HIV capital of China.
Experts say actual HIV/Aids figures are much higher than reported
Some of the country's first cases appeared here and the infection rate is one of the highest in the country.
It is also one of the most progressive in addressing HIV education. But even here, there was embarrassment and denial when we talked to officials about the sex workers who were clearly visible on the town's streets.
When I went to see a pioneering sex education class for teenagers - a controversial concept in China - the teenagers collapsed in embarrassed giggles and hoots of laughter when asked basic questions about puberty and dating.
When I asked some of them later if their parents had ever talked to them about sex, they looked horrified at the very idea.
Acute embarrassment, censorious attitudes from figures of authority - from officials to parents - and a lack of medical confidentiality; these all mean that sexual behaviour is difficult to assess and sexual health hard to track.
Many people with sexual transmitted infections are reluctant to seek help at all.
But the warning signs are there. A recent report on syphilis suggested rates are rising at an alarming speed. That is a concern in itself - but it is also a frightening indicator.
As one doctor described it, the spread of syphilis is a metaphor for the spread of other sexually transmitted infections - and untreated syphilis will amplify the spread of HIV as well.