Regular users of ginseng appear to suffer from fewer colds than non-users:

"People who take ginseng suffer substantially fewer colds, research published yesterday showed.

Only one in 10 of those given daily doses of North American ginseng root extract suffered two or more colds during four months including winter, compared with almost a quarter of those taking placebos.
...

Among those who took ginseng, the proportion who caught two or more colds in the four months was 10 per cent and the average number of colds was 0.68.

In the control group, 23 per cent had two or more colds and the average number of colds was 0.93.

Active constituents of ginseng have been shown to improve the immune system by stimulating the production of immunoglobulin - proteins that bind to foreign substances such as bacteria when they invade the body."