Parasitology and the Diversity of Infectious Organisms

Practice vocabulary flashcards covering parasitology fundamentals, modes of transmission, host interactions, parasite life cycles, metabolic classifications, and viral replicative genetics.

Infectious Disease

A disease caused by another organism (an infectious organism) that is communicable, transmittable, or contagious from the perspective of the host.

Parasite

An organism whose name translates from Greek as 'beside food,' living in or on a host (which serves as its food), inflicting harm, and representing an ecological lifestyle requiring a host rather than a taxonomic designation.

Obligate Parasite

An organism that requires a host for all or most of its life cycle (e.g., viruses and roundworms such as Ascaris lumbricoides).

Facultative Parasite

An organism that is not normally parasitic but can survive when it accidentally enters another organism (e.g., the 'brain-eating amoeba' Naegleria fowleri, which typically feeds on bacteria in pond water but can consume human neurons).

Opportunistic Infection

An infection caused by an organism that is often facultatively parasitic and invades hosts with weakened immune systems (e.g., opportunistic infections in hosts with AIDS).

Ectoparasite

A parasite that lives on the outside of its host, typically causing an infestation rather than an infection (e.g., arthropods like mites, ticks, fleas, and mosquitoes feeding on skin and blood).

Endoparasite

A parasite that lives inside the host, categorized as intracellular (within host cells) or extracellular (outside host cells).

Tissue Tropism

The specific tissue type within the host body that a parasite is capable of infecting.

Cell Tropism

The specific cell type within the host body that a parasite is capable of infecting.

Horizontal Transmission

Direct transmission of an infectious agent from host to host via direct contact (e.g., Ebola, Scabies, Mononucleosis, Influenza).

Vertical Transmission

Direct transmission of an infectious pathogen from mother to offspring, occurring either in utero or during birth (e.g., Zika, Cytomegalovirus, HIV).

Fomite-Borne Transmission

A mode of indirect transmission where a pathogen remains infectious on non-living substances, including contaminated soil (e.g., Hantavirus, roundworms, Salmonella).

Direct Life Cycle

A parasitic life cycle that requires only a single host species to be completed (e.g., all obligate parasitic viruses and bacteria, and various nematodes).

Indirect Life Cycle

A complex parasitic life cycle that requires multiple host species to complete, often involving a free-living environmental phase (e.g., Schistosoma flatworms and Plasmodium species).

Definitive Host

Also known as the final or primary host; the host in an indirect life cycle where the parasite reproduces sexually, or the host that is the focus of medical attention.

Intermediate Host

The host in an indirect life cycle where the parasite does not reproduce or reproduces asexually, or the host of non-medical attention (which may or may not act as a vector).

Dead-End Host

A host to which a parasite is not adapted, resulting in no onward transmission to new hosts, though a variety of diseases may arise (e.g., humans infected with Toxoplasma gondii or Ebola virus).

Dicrocoelium dendriticum

The lancet liver fluke, an indirect life cycle parasite that alters the behavior of intermediate ant hosts to climb to the top of vegetation so they can be ingested by grazing herbivorous definitive hosts.

Myrmeconema neotropicum

A tetradonematid nematode that turns the rump of an infected ant bright red to mimic rainforest berries, functioning as an advertisement to induce birds to eat the ant and advance the nematode life cycle.

Tasmanian Devil Facial Tumor Disease

An infectious cancer in Tasmanian devils where live tumor cells are directly transmitted from one individual to another without any underlying viral agent.

Ascaris lumbricoides

An obligate endoparasitic roundworm belonging to the phylum Nematoda that infects the intestinal tract of hosts.

Saprophyte

A decomposer organism (such as many fungi) that absorbs carbon and energy by breaking down dead organic matter, including wood, decomposing fruit, and vegetables.

Protozoa

A paraphyletic group accounting for the majority of eukaryote diversity, defined as true eukaryotes that are not fungi, plants, or animals, responsible for diseases such as malaria, giardia, sleeping sickness, and leishmaniasis.

Candidate Phyla Radiation

A massive, highly diverse superphylum of uncultured bacteria (also known as Patescibacteria) lacking isolated representatives, identified through cultivation-independent metagenomic sequencing.

Phototroph

An organism that utilizes light as an energy source to produce ATP.

Organotroph

An organism that produces ATP by oxidizing reduced organic molecules such as glucose.

Lithotroph

An organism that produces ATP by oxidizing reduced inorganic molecules such as hydrogen ($H_2$) or ammonia ($NH_3$).

Autotroph

An organism that synthesizes its own reduced organic compounds from inorganic carbon sources such as carbon dioxide ($CO_2$) or methane ($CH_4$).

Heterotroph

An organism that acquires required carbon by absorbing or ingesting reduced organic compounds produced by other organisms.

Photoautotroph

An organism that uses light energy to generate ATP and fixes inorganic $CO_2$ via the Calvin cycle (e.g., cyanobacteria).

Photoheterotroph

An organism that uses light to produce ATP via photosynthesis but absorbs reduced organic carbon compounds directly from the environment (e.g., Heliobacteria).

Chemolithotroph

An autotrophic organism that produces ATP via respiration using reduced inorganic molecules as electron donors (e.g., Nitrosomonas using $NH_3$) and fixes $CO_2$ via the Calvin cycle.

Replication Cycle of a Virus

A four-step cycle comprising: 1. Attachment to host cell receptors; 2. Penetration and injection of viral nucleic acid (DNA or RNA); 3. Viral nucleic acid replication using host machinery; and 4. Packaging into viral particles and release, often destroying the host cell.

Virion

A complete viral particle consisting of viral nucleic acid packaged inside a protein capsid and released from an infected host cell.

Capsid

The protein coat surrounding and protecting the nucleic acid genome of a virus.

Viral Envelope

A lipid bilayer membrane surrounding the protein capsid of certain viruses, aiding in diffusion and entry into host cells.

Lytic Cycle

A viral life cycle in which the viral genome replicates inside the host cell until the cell is lysed (burst open) and destroyed to release progeny virions.

Lysogenic Cycle

A latent viral replication cycle where the viral genome persists or integrates within the host cell without immediate lysis, capable of later generating the lytic cycle.

Positive-Sense Single-Stranded RNA (+ssRNA)

A viral single-stranded RNA genome that has the exact same sense as translated mRNA and can be directly translated into viral proteins by host ribosomes.

Negative-Sense Single-Stranded RNA (-ssRNA)

A viral single-stranded RNA genome that is complementary (anti-sense) to mRNA and must be transcribed into positive-sense mRNA by an RNA replicase prior to translation.

Reverse Transcriptase

An RNA-dependent DNA polymerase that transcribes viral RNA into single-stranded DNA, utilized by retroviruses (+ssRNA-RT) and dsDNA-RT viruses.

Viral Reassortment

The exchange and shuffling of discrete genome segments that occurs when two or more segmented viruses co-infect the same host cell.