Ponder the louse. Consider its plural, lice. Try now not to scratch the multiple itches that have just populated your head at the very thought of these near-microscopic insects crawling around in that forest of hair follicles, laying eggs, sucking blood, and generally creeping you out.

The thing is, your head may not be the likeliest place to feel the itch. After all, we’re home not only to the louse, but to lice, plural. As in two genera of lice, and three different kinds. One of those, the pubic louse, appears to trace back to contact between the Homo lineage and the gorilla, but more on that in a bit.

The history of lice and men is a long, itchy story that intertwines co-evolution not only with these blood-sucking little parasites but also with the microbes they carry. In fact, as visceral as our reaction can be to reading the "head lice" note our little darlings bring home from school, the insects themselves are nothing compared to the microbes they can harbor…and transmit to us. The lice and the microbes have driven us to death, to peace, to giving insects enemas. Yes, you read that right. They have, in short, been our co-pilots—or is it our head pilots?—for as long as Homo sapiens have been around.

Lice have been referred to somewhat mellifluously as "heirloom parasites." They aren’t exactly heirlooms you’d want to inherit on Valentine’s Day…or any day, but they’re more than bugs that might be crawling around in your pubic hair. They and the microbes they carry have shaped fairly recent human history, and thanks to their close affinity for us, our hair, and our blood, we can use them to trace our deeper past.

That affinity is no joke. We host insects that are so desperately attached to us that they’d rather starve—and do—than dine on some other host. So when little Susie brings home the head lice letter, humans are the only mammals in the home that need to worry.

Our constant companions

Those lice-alert letters would bemuse our fairly recent ancestors, for whom lice were simply constant companions until about the 19th century. Along with their dancing lessons, the aristocrats of Europe learned the proper etiquette for disposing of one’s lice, absorbing these valuable lessons even into the 17th century. And in parts of the world where clothes stay on for days and hygiene is a tertiary consideration at best, body lice are not uncommon. As for pubic lice, they remain a worldwide presence, you know, down there.

Whether the host was a 17th-century prince or a modern human, there was at least one commonality: These insects, with a single bite, could give you something that could kill you.

Disease transfer

Lice likely benefited in Western culture from the religious-based aversion many had to bathing. Baths were such an anomaly that Queen Elizabeth I’s contemporaries blamed her near-fatal bout with smallpox on her foolish decision to bathe. Indeed, the parasites and their microbial passengers did flourish among the crowded, unwashed, often stressed and underfed masses that collected in villages, jails, army camps, and workhouses throughout the centuries (See application of DDT to a soldier, image left).

One of the best-known of these microbial passengers is the bacterium responsible for typhus, Rickettsia prowazekii (pdf). The name itself tells the story: the two researchers who were working to identify and isolate the pathogen, Howard T. Ricketts (d. 1910) and Stanislaus J.M. von Prowazek (pdf), each died as a result of typhus infection.

Dying from epidemic typhus was not a pleasant way to go. After a brutal onset involving excruciating headache and blistering fever, a rash would arise. The face would darken and swell, and a crazed delirium could send the sufferer leaping about naked and screaming, impossible to subdue. Following this period, which sometimes seemed to subside into a false recovery, the patient would enter a stupor, which gave the disease its name: typhos in Greek means "smoky" or "hazy." Toward the end, gangrene could set in, rotting fingers and toes and driving away caregivers with the horrific stench. Death, when it came, would be a release.

Typhus and history

When we think of history, we usually consider first the people who shaped it. But microbes, with their ability to wipe out millions, have often played the greater role. Typhus permeates human history like the toxic haze that gave it its name, and rarely is its role one of beneficence.

One exception may be the interaction of Edward Winslow (pictured on the right), founder of Plymouth colony, with Massasoit, sachem of the Pokanoket people with whom the colonists had an uneasy peace. The source of that peace may trace to a Massosoit’s infection with typhus, from which he almost died. Winslow, arriving in the village, first gamely scraped the sachem’s furred tongue and then fed him some fruit preserves. Massasoit, feeling much better—which may well have been the misleading interim lull that typhus can bring—showed his gratitude to Winslow by asking the colonist to scrape the tongues of others in his village who were afflicted. Winslow did so, going well beyond any duty of a diplomat before or since, and the upshot was a shaky peace between the native Americans and the European colonists for 40 years.

But normally, typhus was no diplomat. While experts disagree, it may have been the agent in humanity’s first recorded plague, the Plague of Athens, during the Peloponnesian Wars. Pericles, Athens’ leader, decided to enclose the city in 431 BCE in a wooden barrier to withstand a Spartan siege. The only supply route was a single port, and with the siege, thousands of people surged into the town from the countryside, bringing their lice and microbes with them. The 10,000 dwellings of Athens were packed to bursting, and a plague exploded through the populace. Pericles himself died, as did much of his army, and the devastation marked the end of the golden age of Greek culture. That’s the kind of thing typhus can do.

Typhus marched its way through history in the habitus of the human body louse, Pediculus humanus humanus, and the louse itself was not immune to its influence. An infected louse turns red and dies within a few weeks of infection, but not before it bites and infects its human host. Before succumbing, however, the insect can also poop out millions of typhus bacteria, which can hang on clothing like a toxic dust, infecting others who never have a body louse near them.

Indeed, one researcher, Rudolf Weigl (image left), lost two of his fellow researchers to typhus as they were using solutions of typhus-infected poop to develop a vaccine. Part of the process involved infecting healthy lice with the bacteria. To do so, Weigl’s team would give the lice tiny enemas of the poop–water solution. Weigl himself contracted typhus twice, a demonstration that it could infect more than once. He ultimately succeeded in developing a vaccine in the 1930s, although as late as World War II, DDT was being used to fumigate soldiers against lice and other parasites.

In the first decade of the 20th century, Charles Nicolle (image right) sought to burn through the typhus haze and identify where the disease came from. He was the first to realize that lice stuck to clothing were the agents of disease transmission, and he immediately turned to experimentation on, yes, guinea pigs, followed by chimpanzees. In his fervor to develop a vaccine, he injected himself with a prototype. Finding that he did not develop typhus, he then tried out his vaccine on a few hapless children. They did develop typhus. In these days before the Declaration of Helsinki and human subjects research approvals, his only comment was (pdf), "You can imagine how frightened I was when they developed typhus; fortunately, they recovered." Imagine. Nicolle received the Nobel Prize in Medicine in 1928 for identifying the louse–typhus connection.

The award reflected the significance of his discovery. Typhus was one of the greatest killers in human history. It shaped outcomes of war from the 16th century to the 20th, from Hungary and Turkey, and most of Europe and into the New World. In 1526, the French army had to end its siege of Naples because typhus had killed 28,000 of the besiegers in a month. For perspective, the failure to take Naples left Italy and the papacy in Spain’s power. The pope at the time, Clement VII, to avoid angering Charles V of Spain, declined to grant Henry VIII’s divorce (picture left) request from Catherine of Aragon…and the Protestant Reformation was born.

Typhus helped decimate the half-million strong Grand Armée of Napoleon (on the right) in 1812 to a mere 35,000 men and kept the great general from conquering Russia. Many of the dead met their fates not on the battlefield but in louse-infested hospitals packed with dead bodies.

Even into the 20th century, typhus was actively shaping human destiny. After it killed millions in eastern Europe around the first World War, Vladimir Lenin was driven to comment, "Either socialism will defeat the louse, or the louse will defeat socialism." In the end, it was probably more about free-market economy than lice, but still.

As Hans Zissner, author of the seminal work Rats, Lice, and History, noted:


Soldiers have rarely won wars. They more often mop up after the barrage of epidemics. And typhus, with its brothers and sisters—plague, cholera, typhoid, dysentery—has decided more campaigns than Caesar, Hannibal, Napoleon, and the inspectors general of history. The epidemics get the blame for defeat, the generals get the credit for victory. It ought to be the other way around.

Human evolution

So, where did we pick up these unwanted blood suckers in the first place? All signs point to a human–ape connection, and "connection" may mean something more tangible than an evolutionary link. Some studies suggest interaction between early Homo species and gorillas, and also between early Homo species and us.

The lice we carry around are sucking lice. That’s pretty self explanatory. Two subspecies we harbor, head lice and body lice, belong to the Pediculus (picture left) genus (Pediculus humanus capitis and Pediculus humanus humanus, respectively). The other species we harbor, you know, down there, is a member of the genus Phthirus. Our closest living relatives, the chimpanzees, harbor Pediculus, as well, while gorillas are home to another Phthirus species.

In other words, we share a genus with each of them.

Sucking lice have been sucking primate blood for at least 25 million years. The big story, though, is what happened about 6 to 7 million years ago, and in the case of the gorilla, even later. Humans and apes are supposed to have parted evolutionary ways at about the 6 million year mark. The Pediculus genus seems to have split at the same time, with Pediculus schaeffi hitching a ride with the chimp lineage, and Pediculus humanus sticking with what would become the Homo line. The gorillas split off a little earlier, maybe about 7 million years ago, and the Phthirus may have done the same, sending Phthirus gorillae with the gorillas (natch), while Phthirus pubis eventually became a human problem.

Things get a bit, well, sticky when it comes to the Phthirus line (Phthirus pubis - picture right), however. The split between the gorilla and human lice seems to have happened around 3 to 4 million years ago, millions of years after the gorilla and human branches parted ways. That means that at the 3 to 4 million year point, human ancestors and gorillas must have had some kind of…contact.

The authors who uncovered this finding note in their paper that "How we might have acquired our pubic lice from gorillas is not immediately apparent, however it would be interesting to know whether the switch was very recent (say less than 100,000 years old) or whether it was considerably older" (Reed et al. 2007). Yes, that would be interesting indeed to know.

There are a couple of ways to explain why we carry around a chimp-related genus and a gorilla-related genus of lice. One idea, felicitously dubbed the "pair of lice lost" model, poses that after apes and humans parted ways, each lineage carried both Pediculus and Phthirus. Somewhere along the way, goes the pair-of-lice-lost idea, chimps dropped Phthirus and gorillas dropped Pediculus. Humans, being the avaricious little lineage that we are, held onto both.

But there’s a problem. This explanation just isn’t stingy, or parsimonious enough, and it doesn’t fit with the fact that the split for Phthirus happened 3 or 4 million years ago. In other words, the explanation that fits better is the "recent host switch" idea, as in it switched hosts from gorilla to us thanks to direct contact. Before anyone starts going all King-Kong freaky, it’s important to note that apparently, pubic contact is not required to explain this transfer from host to host. Predation or overlaps in habitats each species used also could have been a way the lice jumped from gorillas to us.

That transfer might have been possible thanks to a few changes the Homo lineage was experiencing, including loss of body hair. As their hirsute habitat decreased, the Pediculus subspecies that would become the head louse retreated to the head. That may have left a bit of an opening for a Phthirus invasion. Phthirus simply may have encountered the least competition, you know, down there.

Divergence from other Homos

Speaking of competition, the history of Pediculus humanus suggests a bit of fancy business happening between Homo sapiens and archaic Homo species, possibly Homo erectus. The Pediculus genus encompasses two lineages that trace back 1.18 million years. One of the lineages underwent a bottleneck—a severe population reduction—right about the time modern Homo sapiens did, 100,000 years ago. The other lineage, however, spent 1.18 million years isolated from its generic partner, until the two somehow had a reunion and both occur in us today. The only way for them to have gotten back together after that long hiatus was direct physical contact. Happy Valentine’s Day!

OK, the contact had to be direct, but it didn’t need to be sexual. It could have been more of that predation or shared hangout that let the two louse lineages reunite. But this re-pairing of a pair of parasites (pairasites?) in Homo sapiens is intriguing because it’s not the only instance: bedbugs, tapeworms, follicle mites, and a parasitic protozoan also exhibit this pattern of closely related pairs. The irresistible inference is that when the lice lineages reunited through the direct contact of Homo sapiens and archaic Homo, so did the lineages of several other unwelcome guests.


While the evolutionary history of lice helps us trace our deep past, we can also use them to explain more recent developments, such as the advent of attire. Body lice hang out—literally—in clothing, and pinpointing when this Pediculus subspecies separated from its head-bound brethren may have helped pinpoint when Homo sapiens decided to put on some clothes. Fig leaves aside, the question has been wide open, with ranges from 40,000 years ago to 3 million. But recent work indicates a divergence between the Pediculus subspecies at 170,000 years ago. This timing puts clothing far earlier than many estimates and well before humans would have needed them for warmth. It’s also well after humans lost their body hair, so for awhile there, our ancestors were wandering around naked and smooth, until a snake came out from behind a tree and…oh, never mind.


Speaking of the Bible, Egyptian mummies and mummies of other provenances have proved to be a wealth information about the evolution of the human-specific louse. They’ve helped researchers determine three separate lineages of Pediculus, one distributed globally, the other two only in certain regions. And the mummies also tell us a lot about what our ancestors had to deal with. In two Peruvian mummies dating to about 1025 CE, for example, one specimen had 407 lice on its head, while the other had 545.

Indeed, mummified lice are among the "best-preserved human parasites," enthused one researcher, yielding not only adult lice but also nits and even eggs. Specimens from Israel dating back 2000 years were culled from hair combs, while crab lice…otherwise known as pubic lice or Phthirus pubis (the gorilla kind), have been found on South American mummies.

Pubic lice

In honor of Valentine’s Day, I’ve saved the pubic lice for last. For many years, lice experts thought that pubic lice were not a New World problem until the Europeans brought them, along with all of their other Old World threats. Then, in 2002, researchers found Phthirus pubis eggs attached to the pubic hairs of a 2000-year-old Chilean mummy, and adult pubic lice have been found in clothing from a 1000-year-old Peruvian mummy. The lice were stuck to the pleats of some cloth from a female mummy, and the enthusiasm of the authors in describing the louse specimens is worth transmitting in their own words (Rick et al., 2002):


Both are females. The foreleg is quite slender, with a long fine claw. The mid and hind legs are strong with thick claws, and the characteristically compressed abdomen is wider than it is long. … Crab lice are usually attached to the pubic and perianal hairs. It remains unclear why P. pubis and P. humanus were found in the same piece of cloth.

The louse with the "long fine claw," they conclude, likely made its way to the Americas with early human migration.

Human pubic lice, also known as crab lice, can occur anywhere there’s hair, you know, down there, including the pubic and "perianal" hairs. It seems that they can also appear in other hairy places, including the legs, arms, armpits, and face. Yes, face. Crab lice can take up residence in the eyelashes, where they produce a crusty deposit and red, itchy eyes. Red is a Valentine’s color, right?

As is appropriate for a sexually transmitted (usually) organism, the slow-moving pubic louse is known for becoming attached to the same pubic hair for quite a long time. A true romantic, indeed.

References and Further Reading:

Gross, L. (1996) How Charles Nicolle of the Pasteur Institute discovered that epidemic typhus is transmitted by lice: Reminiscences from my years at the Pasteur Institute in Paris. Vol. 93, pp. 10539–10540.

Yong, Z., Fournier, P-E., Rydkina, E., Raoult, D. The geographical segregation of human lice preceded that of Pediculus humanus capitis and Pediculus humanus humanus. (2003) Microbiology: Parasitology pp. 565–574. doi:10.1016/S1631-0691(03)00153-7

Reed, D.L., Light, J.E., Allen, J.M., Kirchman, J.J. (2007) Pair of lice lost or parasites regained: the evolutionary history of anthropoid primate lice. BMC Biology 2007, 5:7. doi:10.1186/1741-7007-5-7

Toups, M.A., Kitchen, A., Light, J.E., Reed, D.L. (2006) Origin of clothing lice indicates early clothing use by anatomically modern humans in Africa. Molecular Biology and Evolution 28(1): 29–32.

Raoulty, D., Reed, D.L., Dittmar, K., Kirchman, J.J., Rolain, J-M., Guillen, S., Light, J.E. (2008) Molecular identification of lice from pre-Columbian mummies. Journal of Infectious Diseases 197(4): 535–543.

Reed D.L., Smith V.S., Hammond S.L., Rogers A.R., Clayton D.H. (2004) Genetic analysis of lice supports direct contact between modern and archaic humans. PLoS Biol 2(11): e340.

Robinson, D., Leo, N., Prociv, P., Barker, S.C. (2003) Potential role of head lice, Pediculus humanus capitis, as vectors of Rickettsia prowazekii. Parasitology Research 90(3): 209–211. DOI: 10.1007/s00436-003-0842-5

Mumcuoglu,, Y.K., Zias, J. (1988) Head lice, Pediculus humanus capitis (Anoplura: Pediculidae) from hair combs excavated in Israel and dated from the first century B.C. to the eight century A.D. Journal of Medical Entomology 25 (6): 545–547.

Rick, F.M., et al. (2002) Crab louse infestation in pre-Columbian America. Journal of Parasitology 88(6): 1266–1267.

Zinsser, H. Rats, Lice, and History. (2008) Transaction Publishers, New Brunswick, NJ. 301 pp.

McNeill, W.H. Plagues and Peoples. (1976, 1998) Anchor Press, NY. 365 pp.

Karlen, A. Disease and Plagues in History and Modern Times. (1995) New York: GP Putnam’s. 266 pp.

Crawford, D.H. Deadly Companions: How Microbes Shaped Our History. Dorothy H. Crawford. (2007) Oxford University Press. 250 pp.

Philbrick, N. Mayflower. ( 2006) Viking Penguin. 461 pp.

Image Credits:: Pediculus humanus, public domain, Wikimedia Commons; Pthiris pubis, public domain, Wikimedia Commons; Rudolf Weigl, Wikimedia Commons; Charles Nicolle, Wikimedia Commons; Napoleon, Wikimedia Commons; Using DDT to combat lice and typhus, Wikimedia Commons; Henry VIII by Holbein, Wikimedia Commons; Human family tree (2003), Wikimedia Commons; Edward Winslow, Wikimedia Commons.

About the Author: Emily Willingham, author of The Complete Idiot’s Guide to College Biology, has a B.A. in English and a Ph.D. in biology, and she’s not afraid to use them, often together. She blogs for EarthSky and at The Biology Files and tweets her interests, random thoughts, and cool stuff from other people as @ejwillingham. In a previous life, Emily was an assistant professor of biology and an actual practicing researcher.

The views expressed are those of the author and are not necessarily those of Scientific American.