Invasion generally involves several steps: 1. Entry: The _pathogen_ gains access to the host through various _modes of transmission_, such as respiratory droplets, contaminated food or water, or vector-borne pathways. 2. Establishment: Once inside, the pathogen must evade the host's _immune system_ to establish infection. 3. Replication and Spread: The pathogen replicates and spreads to other tissues or individuals, facilitated by various mechanisms like direct cell-to-cell transfer or release into bodily fluids.
