Figure 2. Evolutionary layers of the X and Y chromosomes. (a) Comparative mapping of genes from the human X in marsupial mammals detected an ancient region conserved on the X in all mammals and denoted XCR, as well as a recently added region denoted XAR that is autosomal in marsupials.
Poliovirus, the causative agent of poliomyelitis (commonly known as polio), is a human enterovirus and member of the family of Picornaviridae. Poliovirus is composed of an RNA genome and a protein capsid. The genome is a single-stranded positive-sense RNA genome that is about 7500 nucleotides long.
The virus usually enters the environment in the feces of someone who is infected. In areas with poor sanitation, the virus easily spreads through the fecal-oral route, via contaminated water or food. In addition, direct contact with a person infected with the virus can cause polio.aralytic polio will often begin with symptoms similar to nonparalytic polio, but will progress to more serious symptoms such as a loss of muscle reflexes, severe muscle pain and spasms, and loose or floppy limbs that is often worse on one side of the body.