contagion

Short definition from Wiktionary

Noun

1. 1
   A disease spread by contact.
2. 2
   The spread or transmission of such a disease.
3. 3
   The spread of anything likened to a contagious disease: The passing on of manners or behaviour through a closed community or household.
4. 4
   The spread of anything likened to a contagious disease: The spread of (initially small) shocks, which initially affect only a few financial institutions or a particular region of an economy, to other financial sectors and other countries whose economies were previously healthy.
5. 5
   A recession or crisis developed in such manner.