A number of infectious agents and infection related conditions can contribute to hair loss.

Some common ones are described here:

 

Folliculitis

Folliculitis is a term for inflammation of hair follicles.

It looks like acne with little rings of inflammation surrounding the opening of a hair follicle.
In the early stages of a folliculitis, the hair fiber may still be present, but as the folliculitis progresses the hair often falls out.

When folliculitis is severe, inflammation is so intense that it can permanently destroy the hair follicles, leaving little bald patches.
There are non infectious forms of folliculitis, such as those caused by oils and greases applied to the skin that clog up the hair follicles, but folliculitis is usually due to a bacterial infection.
Particularly common is an infection of the hair follicles by Staphylococcus aureus.

 

Piedra

Piedra (trichomycosis nodularis) happens when the hair fibers are infected by a fungus.
The visible indicator of a piedra infection is development of hard nodules on hair fibers. Indeed, "piedra" is Spanish for stone.

The nodules are a concretion of hyphae and fruiting bodies of the fungus, known as an ascostroma, from which the fungal spores are released.  
There are two basic types of piedra: black piedra and white piedra, referring to the color of the nodules formed on the hair fiber.

Black piedra is due to the fungus Piedraia hortae and is mostly found in tropical countries, while white piedra is due to Trichosporon beigelii.
Piedra infection may affect hairs of the scalp, body, and genital areas.
Usually the infection is relatively benign.

In parts of Malaysia, the nodules of black piedra are considered attractive and traditionally women encouraged it's growth by sleeping with their hair buried in the soil.
However, when the infection is severe the fungus weakens the hair fiber, making it easy to break off.

 

Ringworm

Surprisingly, ringworm has nothing to do with worms, but is a fungal infection that can occur anywhere on the body.

If it develops on the scalp, it can cause patches of hair loss and is known to doctors as "tinea capitis."
Ringworm is the same thing as athlete's foot, and the same kind of fungal infection that can affect the nails too.
On the scalp, ringworm usually begins as a small pimple that progressively expands in size, leaving scaly patches of temporary baldness.
The fungus gets into the hair fibers in the affected area and these hairs become brittle and break off easily, leaving a bald patch of skin.
Affected areas are often itchy, red, and inflamed, with scaly patches that may blister and ooze.
The patches are usually redder around the outside with a more normal skin tone in the center.
This may create the appearance of a ring, hence the name, ringworm.

 

Demodex folliculorum

Some people believe Demodex folliculorum contributes to hair loss and that removing it will enable hair regrowth.
But the organism does not cause hair loss.
Demodex is a little worm like creature that likes to live on skin and in hair follicles.
It feeds on dead skin and oils, so it particularly likes to live in hair follicles where there are lots of both.
Humans are born free of Demodex, but during childhood, through contact with others, the skin can become infected with it.

For the most part, we never know they are there.
They are benign, if repulsive, little creatures.
The most common problem with Demodex is that they may cause irritation, particularly in the eyelashes.
If you have itchy eyelashes, Demodex may be the problem.

 

Seborrheic Dermatitis

Seborrheic dermatitis is first and foremost a skin condition, but it can involve infection and temporary hair loss if the dermatitis is located on the scalp or other skin areas.
The dermatitis causes scaly, sometimes oily, inflamed skin that can be itchy or even painful to touch.
This is an inflammatory condition that is not well understood, although there does seem to be a genetic component and Caucasians, particularly of Celtic descent, are most susceptible.
Some newborns develop seborrheic dermatitis when maternal androgens are passed from the mother to the baby across the placenta.
Times of hormone fluctuation, such as during puberty, can activate the onset.
The sebaceous glands attached to the hair follicles begin to produce a very rich form of sebum.
The sebum contains fewer free fatty acids and squalene but increased amounts of triglycerides and cholesterol.
The excess, rich sebum production triggers the proliferation of skin flora.
Although seborrheic dermatitis can involve a proliferation of yeast, seborrheic dermatitis is not infectious you cannot catch seborrheic dermatitis.
Where yeast is involved in seborrheic dermatitis it comes from the affected individual's own skin.
We all have yeasts of various types living on our skin the problem in seborrheic dermatitis is that the yeasts may grow to far greater numbers than normal.