Derivation
variable: swglwks [number of weeks swollen glands] (table[s]: tb_tbx, tb_tbxto2007)
variable: swglmm [swollen glands onset month] (table[s]: tb_tbx, tb_tbxto2007)
variable: swglyear [swollen glands onset year] (table[s]: tb_tbx, tb_tbxto2007)