Fungi are one of the great groups of organisms (and pathogens) that we encounter. They are eukaryotes distinct from both plants and animals (although they are more closely related to the latter). The fungi are a massively diverse group of well over a million species. The 'standard' type of fungus was brilliantly introduced in Richard Dawkins' The Ancestor's Tale thus Mushrooms and toadstools give the wrong impression - these conspicuous plant-like structures are the spore-producing tips of the iceberg. Most of the business part of the organism that made the mushroom is under the ground: a spreading network of threads called hyphae. The collection of hyphae belonging to one individual fungus is called the mycelium. The total length of mycelium of an individual fungus may be measured in kilometres, and may spread through a substantial area of soil.
A single mushroom is like a flower growing on a tree. But the 'tree', instead of being a tall, vertical structure, is spread out like the strings of a giant tennis racket underground, in the surface layers of the soil.
Of course, there is a huge amount of variation in such a bulky cohort. Some groups of fungi (like the yeasts), eshew the mycelium body plan, prefering to make a living as single cells that divide to form a clumpy mass. Some fungi can even flip between these two manifestations, and are called dimorphic fungi.
What the hyphae (or yeast cells) are doing is digesting whatever it is they are burrowing through: dead leaves and other decaying matter (in the case of soil fungi), curdled milk (in the case of cheese-making fungi), grapes (in the case of wine-making yeasts), or the grape-treader's toes (if he happens to suffer from athlete's foot).
(He really is a brilliant explainer, isn't he?) Of course, fungi can be identified in a more formal manner by various criteria, such as having a cell wall of both chitin and glucans, but that's not all that important for us here.
Fungi have proved very useful to human beings over the ages, being variously used to leaven bread, to ferment foodstuffs (thereby memorably generating beer and wine), and to fill our bellies (e.g. mushrooms).
Medically, the minority of fungi that are actually pathogenic are conventionally divided up into two groups:
Superficial mycoses - in which the fungi grows at the bdy surface in skin, hair and nails.
Deep mycoses- in which the fungi attack our internal organs.
As a rule of thumb, the superficial mycoses like athlete's foot and candidal vaginitis are common and far from life-threatening, whereas the deep mycoses, although frequently lethal, are opportunistic infections that require a significant degree of host immunosuppression in order to lodge within us.
When our bodies do come into contact with fungi, they usually throw the innate immune system's phagocytes at them. Neutrophils seem to be particarly important. If the fungus is small enough, it is simply phagocyosed and dealt with in the usual way, but it may still be attacked by extracellular factors produced by the phagocytes if this is not an option. The adaptive immune system also has a role, of course, most notably the T lymphocytes.