How do lice become infected from person to person?

Understanding Lice

What Are Lice?

Types of Human Lice

Human lice are obligate ectoparasites that infest only people. Three distinct species affect humans, each with a characteristic body region and a specific route of person‑to‑person spread.

Head lice (Pediculus humanus capitis) – reside on the scalp and attach to hair shafts. Transmission occurs primarily through direct head‑to‑head contact, especially among children sharing close physical space; secondary spread is possible via hats, brushes, or hair accessories that have recent contact with an infested scalp.
Body lice (Pediculus humanus corporis) – live in the seams of clothing and lay eggs on fabric. Infestation spreads when contaminated clothing or bedding is transferred to another individual, typically in crowded or unhygienic environments where garments are not regularly washed at temperatures that kill lice.
Pubic lice (Pthirus pubis) – inhabit the coarse hair of the genital area, perianal region, and occasionally facial hair. Transmission is almost exclusively through sexual contact; occasional spread can happen via infested towels, bedding, or clothing that contacts the affected hair.

Recognizing these species and their preferred habitats clarifies the pathways through which lice move from one host to another, informing effective prevention and control measures.

Life Cycle of a Louse

Lice transmission between individuals depends on the biological development of the parasite. The life cycle consists of three distinct phases that each facilitate spread.

Egg (nit): Female louse deposits oval, cement‑bound eggs on hair shafts close to the scalp. Eggs hatch after 7–10 days at optimal temperature.
Nymph: Emerging nymph resembles an adult but is smaller and lacks full reproductive capacity. It undergoes three molts over 9–12 days, gaining mobility after the first molt.
Adult: Fully formed adult measures 2–4 mm, feeds on blood several times a day, and mates. Females lay 6–10 eggs per day, perpetuating the cycle.

During the egg stage, lice remain attached to the host, preventing loss during brief contact. Nymphs acquire the ability to crawl across the scalp within a few days, allowing rapid movement to neighboring hair and direct transfer during head‑to‑head contact. Adult lice, capable of sustained feeding and mating, increase the number of viable eggs on an infested person, raising the probability of spread to others through shared clothing, hats, or close physical proximity.

Understanding each developmental stage clarifies why lice are efficiently passed from one person to another and highlights points where intervention—removing eggs, eliminating nymphs before they mature, or eradicating adults—can interrupt the transmission chain.

Transmission Mechanisms

Direct Contact

Head-to-Head Contact

Head‑to‑head contact constitutes the primary pathway for lice to move from one person to another. When two individuals press their scalps together, adult lice and newly hatched nymphs can transfer directly across hair shafts. Eggs (nits) attached to hair strands may also be dislodged and fall onto a nearby head, where they hatch and establish a new infestation.

The transfer process occurs within seconds. Lice do not jump; they crawl. Physical proximity allows the insects to locate a new host by sensing heat and carbon dioxide. Once on a new scalp, they begin feeding within minutes, while nits attached to the hair near the scalp develop over 7–10 days before hatching.

Key factors that increase transmission through direct contact:

Prolonged close contact (e.g., shared sleeping arrangements, sports activities).
Dense or long hair that facilitates lice movement.
Inadequate hair hygiene that leaves nits attached to shafts.
Presence of an active infestation on either participant.

Preventive actions focus on minimizing direct scalp contact and managing existing infestations:

Encourage children to avoid head‑to‑head play during outbreaks.
Use protective headgear (hats, helmets) when close contact is unavoidable.
Conduct regular inspections of hair for live lice and nits.
Apply approved topical treatments promptly when lice are detected.
Wash and dry personal items (combs, hats) at high temperatures to eliminate stray insects.

Body-to-Body Contact

Lice spread primarily through direct skin‑to‑skin contact. The insects cannot fly or jump; they move by crawling from one host to another when bodies touch. Transmission occurs most often when:

heads or hair brush against each other during play, sports, or close social interaction;
a person leans against another’s shoulder, neck, or back, allowing lice to crawl onto the new host;
close contact in crowded living conditions, such as dormitories, shelters, or military barracks, brings bodies into frequent proximity;
shared personal items that remain in contact with the scalp—hats, scarves, helmets, hairbrushes—are passed from one person to another without cleaning.

Off‑host survival is limited to 24–48 hours, so lice depend on immediate transfer. The risk rises when individuals have long hair, dense head coverings, or engage in activities that keep hair in contact for extended periods. Prompt removal of lice and disinfection of personal items interrupt the body‑to‑body transmission cycle.

Indirect Contact

Sharing Personal Items

Lice spread primarily through direct contact, but shared personal objects can also serve as vectors. When a comb, brush, hat, or hair accessory is used by an infested person, viable eggs (nits) or mobile lice may remain attached to the surface. Subsequent use by another individual introduces the parasites to a new host.

Key items that facilitate transfer include:

Hair‑care tools (combs, brushes, hair clips) that contact the scalp.
Headwear (caps, helmets, hairbands) that sit close to hair.
Bedding or pillowcases that have been in contact with an infested scalp.
Towels or scarves that have been draped over hair.

The risk is highest when the item is moist or not cleaned promptly, because lice survive longer in a warm, humid environment. Regular disinfection—soaking in hot water (≥130 °F/54 °C) for at least 10 minutes, or using a lice‑specific spray—eliminates attached organisms.

Preventive practice involves assigning personal grooming tools to each individual, avoiding the exchange of headwear, and laundering shared fabrics at high temperatures. Implementing these measures reduces the likelihood that lice will move from one person to another via contaminated objects.

Contaminated Surfaces

Lice are primarily spread through direct head‑to‑head contact, but contaminated objects can also serve as vectors. When an infested person brushes, combs, or shares hats, the eggs (nits) and adult insects adhere to the fabric or plastic surface. If another individual touches the same item within a short period, the lice may crawl onto the new host’s scalp.

Key points about indirect transmission:

Hair accessories – combs, brushes, hair clips, and hats retain live lice and viable eggs for several hours.
Bedding and pillows – lice survive on sheets, pillowcases, and blankets for up to 24 hours, especially in warm, humid conditions.
Clothing – caps, scarves, and coats that have been in close contact with an infested head can harbor insects.
Personal items – headphones, earbuds, and helmets may contain lice if placed directly on the hair.

The risk diminishes rapidly as lice cannot survive more than 48 hours without a human host. Prompt laundering of contaminated fabrics in hot water (≥ 130 °F) and thorough drying at high heat effectively eliminates the insects. Disinfecting non‑washable items with alcohol‑based sprays or freezing for 24 hours also reduces transmission potential.

Preventive measures focus on minimizing shared use of personal items, inspecting and cleaning shared surfaces regularly, and isolating infested individuals until treatment is complete.

Factors Influencing Transmission

Proximity

Lice spread primarily through direct head-to-head contact. When individuals are within a few centimeters of each other, adult females can place eggs on a host’s hair shafts, and nymphs can crawl onto a new scalp within seconds. Brief encounters that involve close proximity, such as hugging, sharing helmets, or leaning over a shared workstation, provide sufficient opportunity for insects to transfer before detection.

Physical closeness also facilitates the movement of mobile stages. Nymphs, which are less than a millimeter long, cling to hair strands and can be dislodged by friction when heads touch. Adult lice, capable of surviving for 24–48 hours off‑host, may hitch a ride on clothing or personal items that are placed near the infected person’s head. The shorter the distance between hosts, the higher the likelihood that an insect will locate a suitable attachment site before it perishes.

Typical proximity‑related scenarios include:

Children playing together with heads adjacent.
Sports teams using shared headgear (helmets, caps, goggles).
Family members sleeping in the same bed or bunk.
Hair salons where stylists handle multiple clients without changing tools.
Group accommodations (dormitories, camps) where personal space is limited.

Mitigation strategies focus on reducing close head contact and limiting shared items. Regular inspection of hair during activities that involve proximity, immediate removal of personal headgear after use, and disinfection of tools between clients decrease transmission risk. Education on recognizing early signs and enforcing personal space during outbreaks reinforces control measures.

Duration of Contact

Lice spread primarily through direct head‑to‑head contact. Transfer occurs when an adult female or nymph crawls onto a new host and begins feeding. The required contact time is brief; research shows that a few seconds of sustained proximity can be enough for a louse to move from one scalp to another. Longer interactions increase the probability that multiple insects will transfer, but even brief encounters during activities such as hugging, sharing a pillow, or playing in close quarters can result in infestation.

Key points about contact duration:

Seconds to minutes: A single second of head contact may allow a louse to relocate; extended periods (several minutes) raise the likelihood of several insects moving.
Continuous proximity: Situations where heads remain close for prolonged periods, such as sleeping in the same bed, facilitate multiple transfers.
Intermittent contact: Repeated short contacts throughout a day accumulate risk similarly to a single longer encounter.

Understanding the minimal time needed for transmission helps target preventive measures, emphasizing avoidance of direct head contact, especially in environments where close interaction is frequent.

Environmental Conditions

Environmental factors create conditions that facilitate the transfer of head or body lice from one host to another. Warm temperatures accelerate lice metabolism, shorten the life cycle, and increase egg hatch rates, resulting in larger populations on infested individuals. High relative humidity (above 50 %) prevents desiccation of nymphs and adults, allowing them to survive longer on clothing, hair, or bedding.

Crowded settings amplify contact opportunities. When many people occupy limited space, direct head-to-head contact and shared use of personal items become frequent, raising the probability of lice moving between hosts. Items that retain heat and moisture—such as hats, scarves, helmets, or tight‑fitting uniforms—provide microenvironments where lice can thrive and be transferred during brief exchanges.

Key environmental conditions influencing transmission:

Temperature: 25‑30 °C optimal for rapid development; lower temperatures slow reproduction but do not eliminate survival.
Humidity: ≥ 50 % maintains moisture; very low humidity causes rapid dehydration and mortality.
Population density: High occupancy increases contact rates and shared item usage.
Material characteristics: Fabrics that trap warmth and moisture (e.g., wool, synthetic blends) support lice survival longer than breathable cotton.
Cleaning frequency: Infrequent laundering of bedding, hats, or uniforms extends the window for viable lice presence.

Mitigation strategies focus on modifying these variables: maintaining cooler indoor climates where feasible, reducing humidity through ventilation, limiting close head contact in group activities, and enforcing regular washing of personal and communal textiles at temperatures that kill lice and their eggs. By controlling the environment, the likelihood of lice moving from person to person diminishes markedly.

Prevention and Control

Identification and Diagnosis

Lice infestations are confirmed through direct observation of live insects, viable eggs (nits), or viable egg shells attached to hair shafts. Visual inspection of the scalp and hair should be performed under bright lighting, using a fine-toothed comb to separate strands and reveal hidden specimens. Detectable signs include:

Live lice moving quickly on the scalp or hair.
Nits firmly attached within 1 cm of the scalp, often at the base of the hair shaft.
Irritation, itching, or reddened skin caused by bites.

Diagnosis relies on microscopic examination when visual identification is uncertain. Collected specimens are placed on a slide, covered with a cover slip, and examined at 40–100× magnification to differentiate lice from other arthropods. Laboratory confirmation may involve:

Preservation of specimens in ethanol for taxonomic identification.
Molecular analysis (PCR) to detect species‑specific DNA, useful in outbreak investigations.
Assessment of resistance markers when treatment failure is suspected.

Professional assessment should include a history of recent close contact with infested individuals, shared personal items (combs, hats, bedding), and the presence of symptoms in household members. Accurate identification and timely diagnosis are essential for implementing effective control measures and preventing further spread among people.

Treatment Options

Effective management of head‑lice infestations hinges on eliminating the parasites before they can pass to another host. Prompt treatment reduces the probability of re‑infestation through close contact, shared objects, or communal environments.

Over‑the‑counter pediculicides such as permethrin 1 % lotion or pyrethrin‑based sprays are applied to dry hair, left for the recommended duration, and then rinsed. These agents target the nervous system of lice, causing rapid paralysis.
Prescription‑strength agents include malathion 0.5 % lotion and ivermectin 0.5 % cream. They are reserved for cases where resistance to OTC products is documented or suspected.
Oral ivermectin (single dose of 200 µg/kg) offers systemic action, useful for severe or widespread infestations and for patients unable to use topical preparations.
Mechanical removal employs fine‑toothed nit combs. Repeated combing every 2–3 days for two weeks physically extracts live insects and viable eggs, complementing chemical treatments.
Environmental control requires washing bedding, hats, and personal items in hot water (≥ 60 °C) or sealing them in plastic bags for two weeks to kill dormant lice. Vacuuming upholstered furniture and car seats eliminates stray insects.
Alternative options such as dimethicone‑based lotions create a physical barrier that suffocates lice without relying on neurotoxic chemicals; these are appropriate for individuals with hypersensitivity to conventional agents.

Combining a topical or oral pediculicide with diligent combing and thorough decontamination of personal items yields the highest eradication rate and minimizes the chance of onward transmission.

Preventing Reinfestation

Hygiene Practices

Effective hygiene practices directly influence the transmission of lice between individuals. Regular combing with a fine-toothed lice comb removes adult insects and nymphs before they can lay eggs. Washing hair daily with hot water and a mild shampoo reduces the likelihood of infestation, especially after close contact with an infested person. Immediate laundering of personal items—such as hats, scarves, hair accessories, and pillowcases—at temperatures of at least 60 °C eliminates viable lice and eggs. Disinfecting surfaces that come into frequent contact with hair, including headrests, hairbrushes, and combs, further limits spread.

Key actions to implement:

Inspect hair and scalp each morning for live lice or viable nits.
Apply a lice‑preventive conditioner containing dimethicone after each wash.
Store personal headgear in sealed plastic bags when not in use.
Avoid sharing hair‑care tools, helmets, or earphones.
Vacuum upholstered furniture and carpets regularly to remove detached eggs.

Consistent adherence to these measures interrupts the life cycle of lice and curtails person‑to‑person transmission.

Cleaning Belongings

Lice move between hosts primarily through direct contact, but personal items can act as secondary carriers. Proper sanitation of clothing, bedding, and accessories reduces the likelihood that a person will acquire an infestation after contact with an infected individual.

Wash shirts, jackets, scarves, and socks in hot water (≥130 °F) for at least 10 minutes; tumble‑dry on high heat for 20 minutes.
Soak bed linens, pillowcases, and blankets in water above 130 °F, then machine‑dry on high heat.
Submerge hair brushes, combs, and hair accessories in hot, soapy water for 10 minutes; rinse and air‑dry.
Place hats, helmets, and headbands in sealed plastic bags for two weeks, a period sufficient to kill lice and nits without heat.
Clean upholstered furniture and car seats with a steam cleaner set to ≥212 °F; vacuum cushions and discard the vacuum bag afterward.

Items that cannot endure high temperatures, such as delicate fabrics or electronic devices, require alternative methods. Seal these objects in airtight containers for a minimum of two weeks, or submit them to professional dry‑cleaning services that employ chemical treatments known to eradicate lice.

After cleaning, avoid re‑exposure by storing laundered items in clean, sealed containers until use. Regularly inspect personal belongings for lice or nits, and repeat the sanitation process immediately after any confirmed case. This disciplined approach limits indirect transmission and supports long‑term control of head‑lice infestations.