Is there a risk of cross-infection?

Generally, washing socks and underwear together will not cause cross-infection. There are three main reasons for this:

Most bacteria on socks are common skin-degrading bacteria that also exist on other parts of the body. Even if they are transferred, they don't have a significant impact. Furthermore, these bacteria require a closed, moist environment to survive after being transferred, and generally cannot multiply in large numbers on other parts of the body.

If you use a washing machine, the vast majority of microorganisms will be shed from your clothes. Research from the University of Bonn in Germany found that washing clothes in 20.5°C water for 15 minutes can remove more than 95% of microorganisms. Furthermore, drying clothes in the sun or tumble dry after washing further eliminates microorganisms.

Even if a small number of microorganisms survive, our skin will act as a barrier to prevent their invasion.

Therefore, washing underwear and socks together is generally not a big problem.

However, in two special circumstances, it is still recommended to wash them separately!