How or why this might happen remains unknown, but it may be due to competition between different mitochondrial strains within a cell.

ECONOMIST: Trojan DNA

New viruses are formed when two different strains infect the same cell and exchange whole genes - a process known as genetic shift.

BBC: Winter bugs: A yearly battle for dominance

Chiron is racing to finish late-stage trials of a method that grows strains within clones of a 1980s-era dog kidney cell.

FORBES: Broken Eggs