There is a certain species of Cordyceps fungus called the “Zombie-ant fungus” that infects ants and controls their behavior. Infected ants fall down from the forest canopy where they live, then bite onto a leaf near the forest floor and die still attached there. After the ant is dead, the fungus continues to grow inside its body and replaces the ant’s tissues with its own. Eventually, the fungus grows out of the ant’s body, and releases spores into the air. Scientists don’t understand exactly how the fungus controls the behavior of the ants yet, but they do know that the fungus infects the ant’s brain and somehow causes its body muscles to weaken.

The fungus might also manipulate the nervous system and interrupt communication between the ant’s brain and its body. The area near the forest floor where the ant dies is the best place for the fungus to grow and disperse its spores. So, the fungus is somehow forcing the ant host to take it to the place where it has the best chance of infecting other ants.

I doubt we really know how it does it, but the fungal filaments grow into the ant's brain and cause the ant to have impulses that it otherwise wouldn't. They don't really have great control: the ant can't hide the fact that it is infected, for example, and while the ant can be inspired to climb to a high place, it can't find its way back to the colony so easily. It's probably not too different from how addictive drugs can control you.

Thank you, I didn't know about this, and it's quite interesting. I went to google.com and entered your question: "How does the cordyceps fungus control ants"

The first hit was from LiveScience.com, which is a good website. They published an article about me on the site once ;-]

Here's some of what it says:
"A carpenter ant infected with a zombie ant fungus in the genus Ophiocordyceps. [There's a picture here.] Once the fungus kills its hapless drone it grows a spore-releasing stalk from the ant's head in order to infect more ants. ... Or maybe they never encounter the spores."

Here's the website:
here

Although it may seem weird for a parasite to alter an animal’s behaviors, there are common examples of this. For instance, in humans, the guinea worm causes an infected person to feel an intense burning in their leg. They then go into water to relieve the burning which allows the worm to burst out into the water to complete its life cycle.

Another example is rabies which destroys the brain causing the animal to act strangely and aggressively.

Extending these examples to the cordyceps fungus in ants, once the fungus penetrates the ant’s exoskeleton it reproduces. It releases nerve toxins which reduce the ant’s ability to control its muscles, causing it to fall down to the forest floor. This is so that the fungus can reproduce in the more humid environment at a lower height. The ant then begins to climb up a plant and bites a leaf with a high amount of force. The fungus then destroys the ant’s muscles so that it can’t move or remove its jaw from the leaf. Then the fungal stalks grow out of the ant and release spores to infect more ants. This “zombie ants” example is extraordinarily complex compared to other parasitic control and may in fact alter the brain in specific ways.

There is still a lot we don’t understand about how this fungus specifically alters ant behavior and times the attack.