Little is known of the fungal pathobiology of ants, but there are several types of fungi that have been found parasitic on ants.
While many people have had the pleasure of discovering 'Vegetable Caterpillars', that is, Lepidoptera larvae parasitised by fungi of the genus Cordyceps, ants may also be attacked similarly.

The genus Cordyceps (Ascomycetes) is the largest of the entomophagous (insect attacking) fungi and often they are quite highly coloured as well. The mycelium of this fungus pervades the host thoroughly, killing it. This invasive mycelium eventually produces a sporophore shaped like a 'boll' or 'club' on a stalk that can be raised about 10 cm outside the body of the host.

An unusual Cordyceps that invades workers of the ant Camponotus abdominalis, found in Guyana, is C. subdiscoidea which has a disc shaped sporophore borne on a stalk that emerges from behind the ant's head.

This terminal structure contains numerous ascocarps, each containing spores within elongate asci. There is little doubt that it would be an unusual and intriguing experience to find a large ant, dead, yet standing rigidly to attention, with a Cordyceps sporophore raised above it's head like a flag!

Another fungus, Desmidiospora myrmecophila, discovered in 1891, on the ant Camponotus pennsylvanicus (and rediscovered in Arizona, in 1986, on a worker ant of Camponotuus semitestaceus) completely envelopes the ant with much branched and septate hyphae, covering the host in a white flocculent mass, emerging especially from between abdominal segments, covering the insect completely and extending a short distance around the ant.

Perhaps the most specialised of the fungi living on ants are members of the Ascomycetes order Laboulbeniales, remarkable in that they neither kill nor harm the insect host. They are inconspicuous, resembling minute, usually dark-coloured or yellow bristles or bushy hairs, projecting from the outside (integument) of the ant, either singly or in pairs, mostly scattered, and sometimes so dense in certain areas they form a furry coating.

Another fungus, perhaps more serious for the ants, is Aegeritella roussillonensis (Hyphomycetales), that may infest the workers and even the queen of an ant colony, the mycelium sometimes covering the whole body, but most commonly on the ant's mouthparts, seriously effecting the behavioural activity of the ants.

Despite an environment of warm subterranean galleries and a habit of huddling in masses, fungi are not serious parasites of ants and do not play an important role in the demography of ant colonies.

It has been suggested that the fanatical grooming behaviour of ants, the compaction of detritus (including fungal spores) collected on the floor of the mouth cavity (infrabuccal chamber) and the disposal of the resulting 'infrabuccal pellets' outside the nest are the ant's principal means of defence against fungi.

While this housekeeping behaviour is in itself important, ants also spread with their grooming movements a range of potent antibiotics, that are secreted from glands in their bodies, which may act as fungicides.