Fungal skin infections in guineas can be far more serious than parasitic problems if they are not caught early, and treated quickly. As with most things in life, prevention is better than cure, and regular shampooing every 4 - 6 weeks can help to prevent skin problems from happening in the first place.
Fungal skin infections usually start off with "a bit of dandruff" on your guinea's skin. If the hair is gently tugged, it will come out easily and will have dry, flaky skin on the end. This is the stage at which to catch the problem and start treating it before it gets any worse.The Central Nervous System can also be infected by fungal spores, and your guinea may have a fit when you touch the infected nerve endings. All the muscles in the body are affected by the fit and contract randomly. This can include the heart muscle, and if your guinea has repeated fits, the result may eventually be a heart attack and death.
So, catch that "bit of dandruff" early and treat it promptly before you end up with what could be a potentially fatal situation.
If your guinea has a very stubborn fungal infection, there are anti-fungal dips available from your Vet. These are Imaverol and Mycophyte, but do be aware that they are very harsh on the skin. For systemic fungal infections, your guinea may need a course of Itrafungol. Again, see your Vet for treatment.
Ringworm is also a fungal infection that guineas can get, and often appears as a circular pattern of hair loss. This is not often seen in guineas, but great care is required if this has been diagnosed by your Vet as Ringworm is easily spread between animals and humans, humans and humans and humans and animals.