The best replacement for outbreak is epidemic.

The best alternatives to outbreak are rated as following:
★★★★★  epidemic
★★★★★  cluster
★★★  contagion
★★★  breakout
★  mission craze
★  diamond infinite edge
★  blood caked blade
★  razor burn
★  encounter
★  necrosis
★  pse rally
  hot zone
  pse stinger
  ravenous dead
  pandemic

Your alternative for outbreak is not listed? Suggest!
Speaking about outbreak, one probably shouldn't forget about sores.

Other search terms related to outbreak and their ratings are:
★★★★★ sores
★★★ prodrome
★★ herpes
You have in mind a companion for outbreak and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of outbreak in all years. Click on graph for more.
outbreak
epidemic
cluster