### agent | virus:

Phage
* Are virus particles with a head of icosahedral symmetry and a tail of helical symmetry.
* Many phages consist of a DNA genome wrapped in a protein coat
  - have a tail attached to the head
* Most phages are bacterial viruses.
* Some phages occasionally package fragments of host DNA instead.
* allow the bacteria that causes anthrax to find shelter in the guts of earthworms.
* are a form of virus that infect bacteria.
* are among the most complex viruses
  - simplest organisms on the planet
  - described in terms of plaque forming units per milliliter
  - highly specific for the species and strain of bacteria that they attack
  - some of the most studied viruses
  - viruses that attack bacteria
* even allow bacteria to come to the aid of aphids.
* infects a cell, and the DNA becomes integrated into the cell's chromosome.
* integrates into a random site on the host chromosome
  - host genome and becomes prophage
* is used for classification and identification.
* look promising for a number of reasons, starting with their prolific reproduction.
* offer a cheap, safe way to study viruses
  - other advantages over antibiotics
* often have dramatic effects on their hosts.
* replicates as part of host genome.
* usually insert at a specific location when they integrate with a chromosome.

### agent | virus | phage:

Coliphage
* are bacteriophage
  - bacteriophages that infect and replicate in coliform bacteria
  - viral indicators of enteric pathogens in environmental samples
* seem to be excellent alternate indicators of the biological quality of waters.

Lambda phage
* can infect by either of two methods.
* is called a transducing phage.

Lytic phage
* are more suitable for phage therapy.
* produce many copies of themselves.

Phage typing
* method used for detecting different strains of bacteria within a single species.
* useful tool for studying the epidemiology of tuberculosis.

### agent | virus:

Plant virus
* Many plant viruses are also filamentous.
* Some plant viruses are transmitted by aphids
  - have a very limited host range and others attack numerous species
  - infect plants
  - replicate in a limited number of plant cells
* appear to be simple organisms with as few as four genes.
* are fairly simple creatures
  - pathogenic to higher plants
  - ubiquitous in nature infecting both crop plants and weeds
  - viruses that affect plants
* can multiply only within living cells.
* enter their host plants only through wounds, via pollen transmission, or by vectors.
* face special problems initiating an infection.
* have no effect on the health of people or animals
  - specific mechanism for entering the host cell
* offer tools to study plant cell biology and macromolecular transport in plants.
* replicate only within host cells.
* use various mechanisms to replicate and spread from cell to cell.

### agent | virus | plant virus:

Viroid
* are a particular problem with fruit trees
  - common plant pathogens which are a serious economic problem
  - known to cause disease in at least six plant groups
  - responsible for causing serious diseases in many plants
  - small pathogenic RNAs that infect many economically important crops
  - smallest known agents of infectious disease
  - the smallest and structurally best characterised infectious agents presently known
* can cause stunting and back scaling on susceptible rootstock or certain scion combinations.
* have no associated proteins.
* is plant viruses
* mainly cause plant diseases.