The big infected in episode 5 of The Last of Us is called a Bloater. This infected type is extremely hard to find as they take years to form.
It is likely it formed as Henry told Joel that FEDRA drove the infected underground a long time ago.
Episode 5 is the first time we become introduced to this creature, but it is expected we will encounter more of them later down the line.
This zombie type is one of the strongest out of the many forms of infected. However, it may be the most powerful we see in season one.

What Is a Bloater in the Last of Us?
A bloater is a zombie in the fourth stage of the Cordyceps Brain Infection. It can take years for a bloater to form.
They are large in size and very powerful. This is because the long-lasting infection has created layers of thick skin that act as armor plates.
They are extremely aggressive but move slowly and are uncoordinated with their movements. This makes them more predictable than any other infected.
Unlike the earlier stages of the fungal infection, Bloaters will not try to turn humans, instead, they will violently rip their jaw apart with their immense strength.
You can watch the full Bloater scene in The Last of Us Episode 5 below:
Bloaters are sure to cause problems for our two protagonists as their strength was clearly on display with it easily killing Perry, the character which was played by Tommy Miller’s voice actor.

How Can Bloaters Be Killed?
One of the best ways to kill a bloater is with fire. While the fire will not defeat it, it will weaken its armor and allow low-power weapons to deal damage.
Although, even after weakening its outer shell, it will still take quite some time to fully kill one.
In the game, Joel is also able to show that melee weapons are able to harm Bloaters as long as you put enough force into them.
The trailer for episode 6 has launched, and from the looks of things, we may be reintroduced to a character from the first episode!