Head Lice is a common condition which can affect many family members very quickly. It is not a sign of having dirty hair.
It can cause itching of the scalp, but doesn't do any harm to the individual.
The level of infestation can be estimated by how far the white empty nit eggs are away from the scalp.
The most important thing is to only treat with an insecticide if there is actually evidence of an infestation and you only treat the people infected.
Derbac M liquid is a great insecticide as it is water based and is suitable for people with asthma and eczema.