P

Pandemic: A worldwide epidemic of a particular disease.
Papillomaviruses: Viruses thought to be responsible for skin warts only, but a link has now been discovered between certain ones and cervical cancer. There are more than sixty different types of warts viruses, determined according to the area of the body where they infect. Common warts are often found on the hands or knees, and plantar warts (verrucas) on the feet. Genital warts are usually sexually transmitted and found around the moist areas of the genitalia and on the cervix of the uterus. Warts can also develop on vocal cords, and can undergo a malignant change as can genital warts.
Paragonimus: A flatworm in the fluke family that infects the lungs.
Paramyxoviruses: Viruses from the family Paramyxoviridae responsible for such familiar diseases as measles, mumps, parainfluenza, and some respiratory diseases.
Parasitaemia: The presence of parasites in the bloodstream.
Parasite: An organism that derives its nutrients from a living plant or animal, contributing nothing to the survival of the host, and is sometimes a detriment.
Parasitology: The study of protozoa and parasites.
Pasteurella infections: Zoonotic infections that stem mainly from four species in the genus Pasteurella. They can be found in a variety of animals, including the upper respiratory tracts of domesticated dogs and cats, and transmitted to humans from bites or scratches. Onset is rapid with marked swelling and pain at the site of injury, followed by fever and swollen lymph nodes. If treated, the disease will disipate easily, but, if not treated, septicemia, with systemic and life-threatening consequences, often happens.
Pathogens: Microbes that cause infectious diseases and can affect a host sufficiently to cause death.
Pediculicide: An agent which destroys lice.
Pediculosis: An infestation of body and/or scalp lice.
Penicillin: The first anti-microbial drug to be obtained from a mold.
Peptidoglycan: A large complex molecule, found only in bacteria, primarily made up of acetylated sugars, and which forms the rigid structural portion of bacterial cell walls.
Peridontal disease (pyorrhea): A disease of the mouth involving the gums, mouth lining, and bony structures supporting the teeth and caused by plaque formation. It is one of the most chronic diseases in the US and usually caused by any number of bacteria that inhabit the oral cavity. Other contributing factors include a poor diet, improper tooth care, and other infections. Contributing organisms include Actinomyces, and Prevotella melaninogenica (Bacteroides melaninogenicus). They are the leading cause of bad breath and significantly contribute to chronic sinusitis, chronic otitis media, and mastoiditis. Sometimes, gingivitis can be caused by a yeast infection (trench mouth), especially after lengthy periods of antibiotic or steroid therapies. If the gums become infected, the ligaments and bone are also at risk from further infection, and is the main cause of tooth loss in adults. The most dangerous aspect of periodontal disease is the possibility of inhaling a bacterial organism which can then cause more serious infections, involving pneumonia or abscesses. These organisms can also spread into the bloodstream to target other organs, including the heart. Alternative therapies include the use of several herbs that have proven their antibacterial activity. These include thyme, eucalyptus, and peppermint. Methyl salicylate has antiinflammatory activity and is found in meadowsweet and willow. Myrrh has both antiseptic and antiinflammatory properties and has traditionally been used as a mouthwash to treat mouth and gum infections, as well as sore throats. The Japanese use a chewing gum containing eucalyptus, and have found that it does reduce plaque, but the flavor was too strong, so many opted for a mouthwash instead. Listerine is made from compounds found in essential oils: thymol, eucalyptol, methyl salicylate, and menthol, which are proven antibacterial compounds. An alkaloid found in bloodroot (Sanguinaria canadensis) called sanguinarine is becoming a common ingredient in toothpastes and mouthwashes since it, too, has proven to cause a reduction in plaque buildup and gingivitis. Its effectiveness as a mouthwash is better than as a toothpaste. The most common bacterial disease is the one that causes cavities and periodontal disease. Sugary diets are directly responsible for providing ample fuel for the microbes to ferment carbohydrates.
Periplasmic space: The area between the cytoplasmic membrane and the cell wall in Gram-negative bacteria containing certain enzymes involved in nutrition.
Peritrichous flagellation: Having flagella distributed all over the surface of a bacterial cell.
Petechiae: Tiny spots of bleeding under the skin.
Pfiesteria piscicida: A dinoflagellate of the order Dinoflagellida, consisting of minute plantlike marine protozoa that have affinities for both plants and animals. Known as "the cell from hell," they are the suspected cause of a phenomena known as "red tide," which results in the deaths of various marine life and causes severe, sometimes deadly, reactions in humans who eat infected shellfish.
PH: A numerical method of expressing acidity or alkalinity on a scale from 0 to 14. Pure water is neutral at pH 7.0, and the stomach pH is about 3.0. Acids have pH less than 7.0, and Alkalis, greater than 7.0. It is a logarithmic scale so that pH 5.0 is ten times as acidic as pH6.0.
Phage: A bacterial virus that requires a specific host cell receptor site. Bacteria have a limited defence strategy, and phages infect only bacteria. This is accomplished by the phage standing on the surface of a bacterium and locking its "legs" onto a specific cell surface receptor. Phages are organism specific, and these receptors sites indicate the type of bacteria the phage has encountered. The phage then drives a needle-like tube through the bacterial wall and passes its DNA into the bacterium, causing the bacterium to generate new copies of the phage, amounting to approximately 200 new ones every thirty minutes. When the new phage bursts out of the bacterial cell, it kills the bacterium, releasing more new phages in search of more bacteria. Mathematically speaking, if these 200 phages were to infect new bacteria each time, there could well be 40,000 new phages within an hour. Repeating this twice more would give 1.6 billion within two hours after initial contact. It also indicates how few phages are needed to combat a disease. The best part is that when all the bacteria die, so do the phages. Since phages are difficult to handle and must be bacterium specific, this phase of invention was set aside for the more lucritive and easy method of antibiotics. Now, with more and more antibiotic-resistance bacteria, the phage approach is, once again, gaining some popularity -- or at least a second look. Phage treatment against bacterial disease has been used in the Soviet Union for decades, but Western medicine is reluctant to try since pharmaceuticals are more lucrative. One misconception is that there is only one type of phage, and that bacteria can develop a resistance to it. In reality, there are many phages, with each targeting a specific population of bacteria.
Phagemid: A cloning vector that can replicate either as a plasmid or as a bacteriophage.
Phagocytes: Large white blood cells that are a part of the immune response. Their sole purpose is to surround harmful invaders and swallow them. During this fighting, symptoms are produced as a warning to step up prevention. Such symptoms can include fever, chills, loss of appetite, tiredness, inflammation,and rashes.
Pharyngitis: An infection of the pharynx (the upper portion of the throat), usually accompanied by enlarged lymph nodes, erythema, and soreness.
Phenotype: The observable properties of an organism (compare with genotype).
Photoautotroph: An organism able to use light as its sole source of energy and carbon dioxide as its sole carbon source.
Photoheterotroph: An organism using light as a source of energy and organic materials as its carbon source.
Photolithotrophs: Organisms that use light energy and inorganic compounds for growth and carbon dioxide as their sole carbon source.
Photoorganotrophs: Organisms that uses light energy and require a supply of organic compounds for growth.
Photosynthesis: The property of forming organic matter from carbon dioxide, using radiant energy from light. It is the basic growth process of green plants.
Phototrophs: Organisms that obtain energy from light to make their own food.
Phytanyl: B branched-chain hydrocarbon containing twenty carbon atoms commonly found in the lipids of Archaea.
Picornaviruses: Viruses of the family Picornaviradae that are the smallest of the viruses, having two genera responsible for human diseases: Rhinoviruses (common cold) and Enteroviruses (various illnesses).
Pili (singular pilus): Hair-like projections, similar to flagella, but used to attach some bacteria to surfaces or for conjugation and are not associated with motility.
Pinworms (Enterobius vermicularis): Parasitic intestinal worms of the Nematode group.
Plague: A common name for a deadly infectious disease caused by the bacterium Yersinia pestis. It uses rat fleas as an intermediate host before infecting humans. Bubonic plague is characterized by buboes (inflamed, swollen, necrotic lymph nodes). Pneumonic plague is a complication from the bubonic type.
Planaria: Free-living flatworms that live in moist environments.
Plasma: The clear noncellular liquid portion of the blood, primarily composed of water, proteins, salts, and clotting mechanisms, but does not include the red cells or the white cells.
Plasmidx: A small piece of bacterial DNA containing up to 100 genes, but not part of the bacterial chromosome that contains most of the DNA and replicates independently. Plasmids are often found in those bacteria having antibiotic resistance.
Plasmodium: A genus of protozoan parasites noted for causing human malaria.
Platelets: The blood cell elements that aid in the clotting of blood.
Pleomorphism: A phenomena in which bacterial form varies widely, in optimal conditions, even within a single culture.
Plesiomonas shigelloides: An organism not considered part of the normal bowel flora in humans, but has been isolated from both sporadic and epidemic outbreaks of diarrhea.
Pneumocystic carinii: An unclassified parasitic protozoan that causes a lung infection called pneumocystosis. It is emerging as a distinct pathogen for those with AIDS. The trophozoites are inhaled and attach themselves to pulmonary tissue cells causing, death by asphyxiation in the immunocompromised and premature babies.
Pneumonia: An inflammatory lung condition caused mainly by bacteria and viruses in which a fluid exudate collects in the air spaces of the lung, but, occasionally, such other organisms as fungi and protozoans are responsible.
Polio virus: A simple member of the Enterovirus family, being the causative agent of poliomyelitis that attacks the CNS, sometimes causing paralysis of any muscles, including those that control breathing and swallowing.
Polymicrobic: The production of more than one species of bacteria at the same time. Anaerobic infections are commonly polymicrobic.
Polymorphonuclear leukocytes (polys): White blood cells that attack and engulf microbial invaders in the blood stream.
Polysaccharide: A very large sugar molecule composed of any number of smaller sugars called monosaccharides, often used as structural building material by cells.
Portal of entry: The site at which microorganisms gain access to body tissues.
Poxviruses: Viruses that belong to the family Poxviridae, genus Orthopoxvirus; the largest of the viruses causing such diseases as cowpox, rabbitpox, buffalopox, monkeypox, camelpox, raccoonpox, but not chicken pox.
Predome: Any symptom indicating the onset of a disease.
Primary antibody response: First exposure to antigen causing the formation of antibodies mostly of the class IgM.
Prions (proteinaceous infectious agents): Abnormal forms of proteins and the smallest replicating agents causing scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle, transmissible encephalopathy in mink, and, in humans, Creutzfeldt-Jakob disease (CJD), kuru, and fatal familial insomnia (FFI). Prions are considered to be subviral particles highly resistant to heat, disinfectants, etc. Having neither RNA nor DNA, they slowly replicate like no other infectious agent.
Probiotics: A term describing certain bacteria vital to the wellbeing of the intestinal tract. The three most beneficial strains are: Lactobacillus acidophilus, Bifidobacterium bifidum (in adults), B. infantis (in infants and young children), and Lactobacillus bulgaricus.
Proglottids: Segments of a tapeworm that contain both the male and female reproductive organs.
Progressive multifocal leukoencephalopathy: A neurological disease associated with the Papovavirus JC, in which numerous white plaques of demyelination occur throughout the brain, leading to increasing neurological damage and death.
Prokaryote: The class of living things consisting of cells without nuclei. (contrast - eukaryote)
Prophylaxis: A prevention or protective treatment as in the use of an antimicrobial to prevent infection from occurring.
Proteins: Large molecules made up of hundreds of different amino acids, forming the basic structure of cells and enzymes and playing a fundamental role in life.
Proteus, Providencia, and Morganella: Bacteria genera of Enterobacteriaceae all closely related and found in water, soil, sewage, and the intestinal tracts of humans and animals. They often cause of urinary tract infections and infections of burns and are responsible for about 10% of the nosocomial (hospital-acquired) infections.
Protoplasm: The living contents of cells.
Protozoa: A subkingdom of unicellular eukaryotic microorganisms, consisting of some 50,000 species that are larger than bacteria and include both free-living forms and parasites. Their mechanisms of movement can either be by flagella, cilia, or amoeboid, and are major factors in their classification.
Pseudomembrane: A combination of microorganisms, damaged epithelial cells, fibrin, and blood cells, resulting from infection of a mucosal surface.
Pseudomonas: A genus of bacteria made up of more than 300 species that cause major infections. The development of P. aeruginosa pneumonia or bacteremia can produce mortality rates reaching 70%.
Pseudopodia: Temporary footlike projections of cytoplasm associated with amoeboid movement.
Psittacosis: A lung infection cause by the bacterium Chlamydia psittaci, which is acquired by inhaling dust from infected parrots (psittacines) and other birds.
Psychrophile: An organism able to grow at low temperatures and showing a growth temperature optimum of less than 15°C (59°F). Listeria monocytogenes is able to grow in temperatures as low as 0°C and can recover from heat damage during the chilling process. Listeria is not the only psychotroph able to grow in cold temperatures. Others include: Clostridium botulinum type E found mainly in fish; Yersinia enterocolitica, found mainly in pork products and milk, affecting children most often causing unnecessary appendix removal; E.coli; and Aeromonas hydrophilia. (contrast - thermophile)
Purpuric fever: An infectious syndrome in which the patient develops fever and patches of bleeding under the skin, a hemorrhagic fever.
Purulent: Associated with the production of pus.
Pus: A thick yellow or greenish liquid containing blood cells and dead cells formed at the site of an infection or inflammation.
Pustule: A pus-containing blister.
Putrescence: The foul smell caused by the decomposition of tissue.
Pyoderma: An infection of the skin by a pus-producing (pyogenic) bacterium.
Pyogenic: The formation of pus or causing abscesses. Pyogenic cocci is a name used to refer to Staphylococcus aureus and Streptococcus pyogenes because of their ability to induce pus formation. Pyrogenic exotoxins are toxins produced by group A strep and by Staphylococcus aureus that cause fever, a scarlet fever rash, and the toxic shock syndrome that may follow infections with these microorganisms.
Pyrogenic: Fever-inducing.
Pyuria: The presence of pus in the urine.