At the beginning of the 20th century, head lice were seen as the scourge of the working class. Statistics from the School Board of Glasgow at the time give an intriguing insight into the relationship between poverty and head louse numbers.
The principal industry was shipbuilding. Between 1910 and 1930, whenever there was a drop in shipbuilding there was an increase in head lice. The peak came two or three years into the recession when times were toughest.
But the association of head lice with the working class ended in the late 1940s with chemical control methods. And today you are never too posh to get head lice.
See objects of Tudor hygiene such as pomanders and nit combs in Wessex Archaeology's Tudor Seafaring Teacher's Pack.
Travel through time and live like nits in sidriver's comedy sketch, A Brief History of Nits.
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8000BC: Nit found on a human hair of this date in Brazil.
3000BC: Nits found on an Egyptian mummy of this date.
AD350: Aristotle (pictured left) writes theory of how lice occur.
AD72: A comb with a louse attached from this date found at Hadrian's Wall, Cumbria.
AD79: Mount Vesuvius erupts. A louse egg is later found on the head of a woman who died in the disaster.
4th, 5th and 6th century: Combs with lice from this era found in tombs in Egypt.
16th century: The Mary Rose sinks in the Solent with nit combs among the salvage (pictured right).
17th century: The Navajo people of the southwestern US leave behind bevelled combs.
1758: Modern scientific name for human louse, Pediculus humanus humanus, coined by Swedish naturalist Carl Linnaeus; nine years later, Charles De Geer distinguishes the head louse as a separate species, Pediculus humanus capitis.
1842: Henry Denny writes Monographia Anoplurum Britanniae, first major monograph on lice in English.
1939: Insecticidal properties of DDT discovered.
1977: Permethrin manufactured as head louse treatment.
1995: Evidence of pyrethroid resistance grows.
2005: Comb and conditioner elimination method found to be 57 per cent successful, compared with 13 per cent success when using synthetic, chemical-based insecticide preparations.
2007: Metal-tined brand-name comb shows a greater efficacy in removing nits than a plastic one.
Adapted from The Little Book of Nits, published by AC Black.