Head lice have not been shown to transmit any infectious agents and rarely cause any harm. Adults more than children do suffer from considerable outrage at the presence of these parasites. This outrage usually outweighs any public health significance head lice may present. The lice's saliva and faeces may cause some irritation, though rarely. This irritation may, in turn, cause scratching and thereby increase the chance of a secondary infection. However this is rare. The most likely harm caused by head lice is from inappropriate use of chemicals in an attempt to treat them. The continued application of chemicals to the scalp can cause severe reactions on some heads. Parents, in their frustration, can resort to applying products not tested for human use and not shown to have any effect on reducing head lice.