A survey on sexual activity among teenagers who are hanging around in Tokyo's Shibuya-ku found that one in 17 had been infected with a sexually transmitted disease (STD) in the past.

The survey was conducted by "Stop! STD o Kangaeru Kai," a group formed by people in the medical profession to raise awareness about sexually transmitted diseases.

"This is data from a group thought to have an extremely high rate of sexual activity. However, their knowledge on STDs was insufficient, and there is a danger that diseases could be spreading without them knowing it," a group representative said, commenting on the results of the survey.

The survey was conducted on high school students and teenage high school graduates. Sixty-eight percent of the respondents had experienced sex before, and 5.8 percent had contracted a sexually transmitted disease in the past, the survey showed.

The average number of sex partners in total was 5.2, but restricted to those who had contracted a sexually transmitted disease in the past, the average was 37, more than seven times higher.

Over 80 percent of respondents said they could explain what a sexually transmitted disease was, but only about 20 percent had correct knowledge on STDs, such as the fact that it was 10 times easier to contract a sexually transmitted disease than to have an unwanted pregnancy, or that chlamydia, when left untreated, could result in women not being able to have children. More than half of the respondents did not know that AIDS was spreading among young people.

The survey was performed on the street and other locations in Shibuya between Aug. 10 and 16. Responses were obtained from 466 people.

"Sudden AIDS" is a feature of AIDS contraction in Japan. Even though the HIV virus lies dormant in the body for months to years, there is so little interest in being tested for it in Japan that many people don't know they have contracted it until they actually develop fully blown AIDS.

The human immunodeficiency virus, or HIV, works by taking over the immune system, which opens up the opportunity for all sorts of diseases and malignant tumors to arise in the disease called AIDS. According to the 2006 government report on AIDS, heterosexual sexual contact was the most common way for people in Japan to develop AIDS from 1985 to 2006, accounting for 42 percent. Homosexual and bisexual sex accounted for 28.3 percent of cases over the same period, injections 0.8 percent and mother to baby transmission 0.4 percent. Nearly all the AIDS cases in this country are the result of sexual transmission.

Even after somebody has contracted HIV, they can't find out right away. What that leads to is people going around spreading the disease unknowingly for longer periods of time. This can go on for months or even years. They then get sick, go to a hospital for treatment and discover they have got AIDS. That's what makes me scared, because the number of Japan's known HIV patients is too small for the number of its AIDS patients. AIDS experts say that the number of reported HIV carriers in Japan is only the tip of the iceberg and that the actual figure could be a shocking three to seven times more than what is currently known.

There was a massive increase in people taking HIV tests at public health centers in 2006 to 116,000 people, possibly thanks to a widespread public advertising campaign informing people that they could be tested for HIV antibodies at such facilities on weekends, at night, for free and anonymously. But I think the situation still needs improvement and people could be more active in undergoing HIV tests. The media makes a big fuss about AIDS around World AIDS Day on Dec. 1, but otherwise couldn't care less about it. But with AIDS, early discovery and early treatment is essential to make sure that it doesn't become the deadly disease that it can be. Giving people more opportunities to take an AIDS test is an important concern that public health authorities are tackling.

While it is a fact that the number of people who discover they have contracted HIV during a public health center is increasing, at the 18th Japanese Society for AIDS Research meeting in Shizuoka, officials from the Tokyo Metropolitan Komagome Hospital showed that 75 percent of HIV contraction discoveries were made at medical institutions, 9.3 percent at public health centers and 3.4 percent after having donated blood. The report showed that many HIV cases are only discovered at medical institutions after the patient had fallen ill or pregnant, was preparing for an operation, or some other reason. They discovered something was happening with their body, sought medical treatment and found out they had the disease. That's the phenomenon I refer to as Sudden AIDS.

With Sudden AIDS, there are times when the delay in treating the disease means that survival is no longer a possibility. And the cost of treating the disease becomes much higher than it would have been had it been detected earlier. To counter these problems, it is vital to discover the afflicted as quickly as possible before the disease worsens. (By Dr. Kunio Kitamura, special to the Mainichi)

I recently received a letter that made me laugh, wondering whether the world had changed so much that people had become so sexually aggressive or that my column was starting to become useful for people. Here's the letter.

"I'm 29 years old and in the third year of being a housewife. I've got one child. I've known my husband for about 10 years, but for some reason I just don't seem to match with my husband's semen. Inside my vagina it's always itchy. And it gets even itchier if we have sex after my husband has been drinking. I've talked to several of my friends about this problem, and nobody else is going through anything like it. Is it possible our body fluids just don't get along together?"

The symptoms the woman described in her mail appear to be a case of semen allergy, an extremely rare affliction.

Semen allergy's symptoms include painful twinges in the genitals, genital soreness, hot genitals and red blotches like a rash that appear in the external areas of the sex organs. At times, a semen allergy can be discovered following sex without a condom. Putting it simply, using a condom from go to whoa is the best way to prevent a disease and thus be an effective treatment at the same time. Prevention is the best medicine.

But, what caused the semen allergy? Semen is made up of seminal fluid and prostate fluid and has sperm swimming inside it. Some recipients of ejaculated sperm (in this case, it was the woman's body) treat the presence of semen as an invasion by a foreign object. The most common explanation of why semen allergies occur is that the body that receives the semen treats it as a foreign object and reacts to try and expel it by bringing about an allergic reaction. Plainly speaking, this is either an immunological reaction or an antigen-antibody reaction. When the body's immune system brings about a reaction, it can sometimes lead to the symptoms that the woman who mailed me described.

If women experience ailments such as those described, they can seek the aid of a gynecologist, but because so few people -- sufferers and medical practitioners alike -- know much about semen allergy, there is a high likelihood of misdiagnosis, with some women being told such things as their ailment resulting from having a partner's penis that is too big for their vaginas, or having allowed insertion to occur before she is sufficiently lubricated. Some physicians may only look at the local reaction and put women through tests for such diseases as candidal vaginosis, vulvitis, genital herpes or chlamydia. While it is very important to check whether there is some sort of illness there, if the cause of the ailment is a semen allergy, then no amount of treatment for a sexually transmitted disease is going to bring about a cure.

Semen allergies can also affect more than just the genitals. A semen allergy can bring about the same reactions as hay fever, with coughing, sneezing, rashes all over the body and itchiness among its unexpected effects. Normally, these reactions would appear about 20 to 30 minutes after ejaculation and can last anywhere from hours to days. Semen allergy can also be deadly if it leads to what's called an anaphylactic shock, which is typified by breathing difficulties and plummeting blood pressure.

A simple check for a semen allergy involves applying semen to a soft part of the skin, say on the upper arm after scratching the skin, then waiting for about 20 to 30 minutes to see whether it causes redness or a rash to appear. I recommend anybody who suspects they may be allergic to give the test a try.

As I mentioned before, the best prevention and treatment for semen allergy is to use a condom at all times. Doing so, however, eliminates the possibility of pregnancy. If a couple finds themselves in the position where a woman has a semen allergy, many elect to go through the in-vitro fertilization process. (By Dr. Kunio Kitamura, special to the Mainichi)

NOTE TO READERS: Dr. Kitamura's regular "Slow Life, Slow Sex" column, which was supposed to run today, is unavailable this week. Today's story originally ran in Japanese on Oct. 1, 2004. "Slow Life, Slow Sex" and Dr. Kitamura's regular consultation column will appear on the site again as normal next Sunday, Nov. 18. We thank you for your understanding.