The Infected are enemies in The Last of Us.
The results of a parasitic spore making contact with the human brain, the Infected are about as much plant as person at this point.
When the fungus makes contact with a person, it spreads inside of them until it reaches the brain. Here, it extends probes inside of the brain and gains control of its host, mutating them and growing out of them in disgusting, plant-like protrusions in the process.
The fungus in The Last of Us was inspired by the real fungus called Ophiocordyceps camponoti-balzani, which infects ants, controls their minds, and causes plant-like stalks to grow from their skulls.