They emerge when microorganisms find new paths by which to enter susceptible hosts, like the hantaviruses from forests, or when new test methods are developed which enable previously unknown pathogens to be identified.
何美鄉指出，大部份的新興傳染病，致病的原因多不是出現新的致病原，而是微生物有了新途徑進入易感性宿主，像叢林裡的漢他病毒；或者是由於檢驗技術的進步，至今才發現致病的原因。