It's not about 小（弱）. It works the same way as vaccines to our immune system

送交者: Latino2 于 2009-04-02, 15:57:56:

回答: 拉老，你是否认同由湘女于 2009-04-02, 15:42:23:

Sometimes it happens. A new race of virus pop out, and the vaccines can not respond.

But overall, this situation is rare.