Syphilis and HIV are sexually transmitted diseases that are transmitted mainly through sexual contact.From interviews in Sweden, the United Kingdom, Zimbabwe, and Burkina Faso, it is known that the distribution of the number of people of the opposite sex who have sex contact follows the power distribution (Note).This property, also known as scale-free, means that many people have sexual contact with a small number of people (less than a few), but some have extremely large sexual contact networks. ..When an infectious disease is transmitted through a scale-free network, it is easy to spread even if the probability of infection is very low.It is an important issue for public health, but the current situation in countries other than the previous four countries was unknown.
　
　The research team conducted an Internet survey and answered from 2,500 men and women living in Japan (5,000 in total) about "the cumulative number of sexual contacts in life so far" and "the cumulative number of sexual contacts in the last three months". Obtained.As a result, it was found that both have scale-free properties regardless of gender.It was also confirmed in Japan that men declare more sexually experienced people than women.
　
　This study revealed that the sexual contact network is scale-free for the first time in Asia, showing that it is highly universal worldwide.This study is expected to be useful in predicting the spread of sexually transmitted diseases in Japan in the future.

Note: Follow the power law in one of the probability distributions.The mode / median, which is larger than the normal distribution, is biased toward the edge of the distribution and has a long tail that gradually decreases.

Paper information:[PLoS ONE] Is the network of heterosexual contact in Japan scale free?