NARRATOR: As experts begin to search for other explanations,

they turn their attention to a series

of disturbing discoveries that occurred

in the months following the June 2012 attacks.

We had some dolphins that washed up already dead.

We weren't really sure what it was,

but we did hear from other states

that they were having the same issue.

NARRATOR: In fact, it turns out that over the course of 2013,

a total of 48 dolphins are found dead on South Carolina beaches.

Researchers concluded the dolphins were inflicted

with the morbillivirus.

This deadly disease suppresses the dolphin's immune system

and causes inflammation of the brain.

The result is a bizarre shift in behavior.

Infected dolphins have been witnessed

swimming in strange patterns, almost like they are drunk.

And additional research has shown

that brain inflammation in zebrafish

caused them to become more agitated.

So in 2012, could the sharks of Myrtle Beach

have been inflicted with a disease that

changed their behavior, making them

more likely to bite swimmers?

According to Dr. Dan Huber, it comes down

to a shark's immune system.

DAN HUBER: For animals, the skin is

the first line of defense against any type

of an infection.

So the skin is part of the immune system, essentially.

Looking at a section of shark skin which shows what

are called dermal denticles, which

are basically little teeth that make up the scales of a shark.

NARRATOR: The denticles perform a unique function

in the shark's immune system.

DAN HUBER: Dermal denticles creates

a very uneven shape that bacteria

have a hard time adhering to.

Because the bacteria can't adhere to it,

they can't form colonies.

And this is something that helps sharks

to resist bacterial infections.

NARRATOR: And there's also another organ that helps

protect sharks from disease--

their liver.

The liver produces a chemical compound called squalamine.

The squalamine is dispersed inside the shark's cells.

If a virus invades the cells, the squalamine

prevents it from multiplying so the virus doesn't spread.

And the shark doesn't get sick.

Scientists are even testing squalamine to fight cancer

in humans.

This remarkable immune system means

that sharks might be one of the most

disease-resistant creatures on Earth.

Any suggestion that the sharks could have become ill due

to a virus or a bacterial infection

isn't very likely because sharks have

very strong immune systems.