The human body carries fungi not only on the skin, but also under the nail edges. A fungal nail infection can only occur when the fungi penetrate through the nail. Fungi are parasitic plant organisms; it can live in a humid and warm environment without sunlight. Perspiring feet, badly ventilating socks and tight shoes stimulate fungal growth. Other factors that may increase the risk of infection are damaged nails or any wound lesions, individuals afflicted by chronic diseases such as diabetes, immune deficiency disease, vascular circulation problem and psoriasis.

Nail fungus is a common condition that begins as a white or yellow spot under the tip of your fingernail or toenail. As the fungal infection goes deeper, nail fungus may cause your nail to discolor, thicken and crumble at the edge. It can affect several nails.

You may have nail fungus if one or more of your nails are:

1. Thickened
2. Whitish to yellow-brown discolouration
3. Brittle, crumbly or ragged
4. Distorted in shape
5. A dark colour, caused by debris building up under your nail
6. Slightly foul odour

*Nail fungus can affect fingernails, but it's more common in toenails.

Yes, it can spread from one nail to another.

1. Humid and warm environment: sweating heavily/perspiring feet, bad ventilating socks and wearing tight shoes
2. Having a history of athlete's foot
3. Walking barefoot in damp communal areas, such as swimming pools, gyms and shower rooms
4. Having a minor skin or nail injury or a skin condition, such as psoriasis
5. Being older, owing to reduced blood flow, more years of exposure to fungi and slower growing nails
6. Having chronic diseases such as diabetes, circulation problems or a weakened immune system

Ref: https://www.mayoclinic.org/diseases-conditions/nail-fungus/symptoms-causes/syc-20353294