- Larvae
- The earliest stage of a newly hatched worm-like insect.
- Larvicidal
- An agent which prevents and kills larvae.
- Lateral cervical puncture
- A procedure in which a needle is inserted into the neck below the ear to obtain cerebrospinal fluid or to administer drugs near the base of the brain.
- LCM (lymphocytic choriomeningitis)
- A naturally occurring and lifelong viral infection of mice and hamsters. Infected humans may develop meningitis.
- Legionellosis (Legionnaires disease)
- The disease produced by the bacterium Legionella pneumophila from the bacterial family Legionellaceae, which has only one genus, Legionella. There are about thirty known species existing around the world, and at least nineteen of which can cause pneumonia in humans.
- Leishmania donovani, L. tropica, and L. braziliensis
- Pathogenic parasitic protozoa transmitted by blood-sucking sandflies (Phlebotomus), producing two types of disease: ulcerated skin or mucosal lesions, known as cutaneous leishmaniasis, and a more serious infection called visceral leishmaniasis, which involves internal organs.
- Leprosy
- A disease caused by the bacterium Mycobacterium leprae, sharing many common characteristics with the tuberculosis bacterium M. tuberculosis. A nose blow from a person infected with leprosy can discharge ten million bacteria. Mycobacterium leprae causes various symptoms, depending on the type, including lesions in any of a variety of tissues that commonly include the skin and/or the destruction of nerves in the peripheral nervous system.
- Leptospirosis
- A rare bacterial disease contracted after contact with animal urine that is infected with leptospires; spiral-shaped bacteria (spirochetes) also responsible for syphilis and Lyme disease. Scientists have identified over 200 different types of leptospires that are responsible for mild to serious illnesses. Initial illness is often mistaken for viral meningitis or hepatitis. An outbreak at Fort Bragg, North Carolin, in 1942 affected servicemen who had been swimming in freshwater ponds and streams contaminated by livestock urine. Long before, the disease had been noted among sewer workers, miners, farmers, and fish handlers. It was common in Japan, with high mortality rates. It is now known that a wide variety of animals urinating into areas of potential human contact can cause the infection. Such animals including all manner of livestock, mice, dogs, bats, deer, foxes, raccoons, rabbits, pigs, goats, birds, frogs, snakes, and fish can spread the organism into the environment where direct skin contact will transmit the organism. When leptospirosis is severe, it is called Weil’s syndrome after the German physician who first described it in 1886 when it caused liver and kidney problems and meningitis.
- Lesion
- A wound, injury, or circumscribed alteration of tissue caused by an infectious organism.
- Leukemia
- A malignancy in which there is uncontrolled division of abnormal blood cells.
- Leukocidin
- A toxin produced by staphylococci that kills phagocytes.
- Leukocyte
- A white blood cell (usually a phagocyte) responsible for fighting disease.
- Leukocytosis
- A significant increase in the number of white blood cells caused by infection.
- Leukopenia
- An abnormal reduction in the number of white blood cells.
- Lichen
- A fungus in symbiotic union with an alga or a cyanobacterium.
- Ligature
- A suture; a material (often silk) used to close an open wound, commonly referred to as ‘stitches.’
- Listeria monocytogenes
- The only pathogenic bacterium of the four species in the genus Listeria. It causes an illness known as listeriosis which is much more common and just as serious as diphtheria. The organism is capable of growing at temperatures of 0°C (32°F) and lower.
- Lophotrichous
- Having a tuft of polar flagella.
- Lower respiratory tract
- The trachea, bronchi, and lungs. The upper respiratory tract is comprised of the nasopharynx, oral cavity, and throat.
- Lumbar puncture
- Commonly called a “spinal tap;” a procedure in which a needle is inserted into the small of the back to obtain cerebrospinal fluid for tests or to administer drugs.
- Lyme disease
- A disease caused by the spirochete Borrelia burgdorferi and transmitted by the tick belonging to the genus Ixodes. Similar to the pattern of congenital syphilis, the B. burgdorferi spirochete can be passed to a developing fetus. Early stage of the disease is characterized by flulike symptoms and a circular rash (erythema migrans), which is distinct, allowing for easy diagnosis of the disease. The rash will last from two to four weeks, and often expands in a circular fashion out from the site of infection, giving the appearance of a target with a bull’s eye. Sometimes, the disease will not appear until a month later and, sometimes, not for two years after early symptoms disappear. This latent stage is associated with neurologic, cardiac, and/or arthritic symptoms.
- Lymph
- A colorless fluid that contains immune cells that pass from tissues to lymph nodes and then to the blood.
- Lymph vessels
- Channels carrying lymph from the tissues to lymph nodes and back to the blood.
- Lymphadenopathy
- The enlargement of the lymph nodes.
- Lymphangitis
- An inflammation of one or more of the lymphatic vessels.
- Lymphatic
- Pertaining to the lymphatic system, which is comprised of a system of nodes and vessels that transport lymph, a clear yellowish fluid that carries white blood cells.
- Lymphoid
- The system referring to the total mass of immune cells (lymphocytes, macrophages, etc.) in the body. If they were all in one piece, the size would be that of a football.
- Lymphocytes
- Consist of two kinds of cells from the lymphoid system: B cells, which mature in the bone marrow, and T cells, which mature in the thymus gland. Both produce antibodies in order to destroy invading bacteria, viruses, etc. Atypical lymphocytes are abnormal and usually large with a foamy nucleus.
- Lymphogranuloma venereum
- A rare and serious venereal infection caused by Clamydia trachomatis.
- Lymphotropic
- Having a preference for lymphoid cells.
- Lysis
- The rupture of a cell, resulting in the loss of the cell contents, signifying its death.
- Lysogenic
- A bacterial cell which has the DNA of a bacteriophage integrated into its own chromosome. Under these conditions the cell does not produce new bacterial viruses, but may carry out functions under direction of the virus DNA.
- Lysosome
- A sac present inside phagocytes, containing defensive substances, enzymes, etc.
- Lysozyme
- An enzyme found in tears and other body fluids that can breakdown bacterial cell walls.
