- Haemophilus aphrophilus
- A Gram-negative, rod-shaped bacterium that may cause infectious endocarditis.
- Hafnia
- A bacterium of the family Enterobacteriaceae, associated with bacteraemia and diarrheal cases.
- Halophile
- A microbe capable of growing in salt solutions of concentrations in excess of 3%. Such concentrations are toxic to other fresh-water microbes. Halophilic literally means salt-loving; and extreme halophiles can live in such very high salty environments as the Great Salt Lake or the Dead Sea.
- Halotolerant
- An organism capable of growing in the presence of salt (NaCl), but not requiring it.
- Haploid
- A eukaryotic organism or cell that contains one chromosome complement and the same number of chromosomes as the gametes.
- Haustorium (plural haustoria)
- A specialized branching structure produced by pathogenic fungi to penetrate plant cells to obtain food.
- Helicobacter pylori
- A bacterium recognized as causing peptic ulcers, and once known as Campylobacter pylori.
- Helix
- A spiral structure of a molecule containing a repeating pattern.
- Helper T cells
- Lymphocytes developed in the thymus that stimulate the growth, differentiation, and immune response of B cells.
- Hematuria (also spelled Haematuria)
- The presence of blood in the urine.
- Hemagglutination
- The clumping together of red blood cells.
- Hemagglutination inhibition test
- A serological test used to diagnose measles, influenza, and other viral diseases.
- Hematogenous spread (also spelled Haematogenous)
- The spread of microbes through the bloodstream.
- Hemoglobin (also spelled Haemoglobin)
- The iron-containing protein that carries oxygen and gives red blood cells their color.
- Hemolysins
- Bacterial toxins capable of destroying red blood cells.
- Hemolysis
- The breaking or rupturing of blood cells. Staphylococci often causes hemolysis of the cells in blood agar.
- Hemophilus (also spelled Haemophilus)
- Bacteria that are generally harmless passengers in the upper respiratory system. One strain, H. influenzae type b or Hib, is the major disease-producing species of the genus and the prime cause of meningitis in children under the age of three. In addition to bacterial meningitis, H. influenzae is responsible for other diseases, including epiglottitis, sepsis, cellulitis, pneumonia, and septic arthritis.
- Hemorrhage
- Profuse bleeding.
- Hemorrhagic fevers
- Highly infectious fevers complicated by spontaneous, uncontrolled bleeding. Hemorrhagic lesions can occur both internally and externally. The skin lesions often appear as red spots or blemishes, which frequently become dark blue or black involving large areas. Hemorrhagic fevers cause the body’s fluids to leak out of the tissues and orifices, killing up to 90% of their victims. The most famous diseases are Ebola and Marburg, but also include yellow fever, Lassa fever, dengue fever, and others.
- Hermetic
- Air tight.
- Heroin
- A highly addictive narcotic drug related to morphine, nicotine, cocaine, and caffeine.
- Hepatitis viruses
- Viruses belonging to a group called Picornaviruses, which resemble the polio virus in structure, which cause various liver infections, ranging from mild to severe. Hepatitis means an inflammation of the liver, but the word is most often used to refer to a specific disease rather than a general condition.
- Herpesviruses
- Viruses belonging to the family Herpesviridae, having more than ninety members. Five causing human infections are as follows: herpes simplex (cold sores and genital herpes); varicella-zoster (chicken pox); cytomegalo (infectious mononucleosis); Epstein-Barr virus (certain cancers); and human herpes 6 (roseola).
- Heterophile antibody
- A type of antibody that reacts with antigens other than those responsible for induction. Human heterophile antibodies will hemagglutinate sheep red blood cells.
- Heterotroph
- An organism that requires one or more organic compounds for growth, and is considered the same as chemoorganotroph; a system of nutrition that requires preformed organic molecules as a source of energy. (contrast – autotroph).
- Histamine
- A small molecule found in some tissue cells that, when released, acts to produce a number of immunologic phenomena; a substance that will promote shock.
- Histoplasmosis
- A fungal respiratory disease (Darling’s disease), endemic to the Central and Eastern US and caused by the soil fungus Histoplasma capsulatum.
- HIV (human immunodeficiency virus)
- A virus responsible for the disease known as AIDS (acquired immunodeficiency syndrome), transmitted by sexual contact, administration of contaminated blood products, childbirth, contaminated needle sticks, and intravenous drug abuse.
- Homoacetogens
- Bacteria that produce acetate as sole product of sugar fermentation or from hydrogen and carbon dioxide.
- Homofermentation
- A fermentation of glucose or other sugar, leading to virtually a single product, lactic acid.
- Homologous antigen
- An antigen reacting with the antibody it had induced.
- Hookworms (Nematodes)
- Small parasitic worm of the family Ancylostomatidae. With hooked mouths, they are able to fasten themselves to the intestinal wall of their hosts.
- Host
- Any organism (including humans) capable of supporting the growth of another organism, being its source of food.
- Host range
- Kinds of hosts that can be infected by a specific microorganism. Host range is determined by receptors on both the host cells and the microorganism. A parasite with a broad host range can infect many kinds of cells.
- Humoral
- Pertains to body fluids.
- Humoral immunity
- An immune response carried out by antibodies circulating in the blood.
- Huntavirus
- A hemorrhagic-type virus that causes an infection contracted by inhaling dust contaminated by rodents carrying the virus.
- Hyaluronidase (Spreading Factor)
- A bacterially-produced enzyme that digests hyaluronic acid which holds cells together, thereby making tissues more accessible to microbes.
- Hydrocephalus
- An accumulation of cerebrospinal fluid within the brain, caused by a blockage in drainage pathways.
- Hydrophilic
- Capable of dissolving in water. Sugar is a hydrophilic compound.
- Hydrophobic
- Not capable of dissolving in water. Oil is a hydrophobic substance. The term is also used in association with rabies, where an infected animal will show aversion to water.
- Hyperimmune globulin
- Contains high levels of antibodies to a spectic antigen. These globulins are prepared by repeatedly immunizing animals or people with the antigen. Blood is then donated, and the globulins are obtained.
- Hyperimmune sera
- Serum with a high antibody titer, prepared by the repeated immunization of an animal.
- Hypersensitivity
- A harmful immune reaction caused by either antigen-antibody reactions or cellular immune processes. (see allergy)
- Hyperthermophile
- A prokaryote that prefers to grow in optimum temperatures of 80°C (176°F).
- Hyphae
- A single filament or thread which is the basic structure of growing fungus. Masses of them are called mycelium. Some hyphae that grow above a surface look like branches of a plant. These are called aerial hyphae or aerial mycelia. Other hyphae grow into the surface to absorb nutrients much like the roots of plants do to gain their nutrients. These are called vegetative hyphae. Many nuclei are contained within the hyphae with crosswalls called septa, located at frequent intervals along the hyphae.
