What causes a toe fungus to spread to the other toe nails on the opposite foot?

Contact. Fungus can be transferred by fomites or non-living objects such as socks, shoes or carpet. It can also be spread if the infected foot comes in contact with the non-infected foot. It is important to let your feet and shoes dry out regularly to avoid a moist environment that fungus likes. There are also fungal treatments that can be found in your local pharmacy.
Proximity. Once you have a fungus on your toenails it can slowly spread to other nails on the same or opposite foot. This is generally not a highly contagious condition but often spreads to other toes because they are in contact with each other.
Contact. It is a fungus and therefore can spread by contact from the source to other areas.

Related Questions

My 20 mth sons big toenail fell off & his other big toe is lifting from nailbed. No trauma, fungus ruled out, nails are flat/spoon. Ideas on causes?

See below. Onycholysis is the separation of the nail plate from the nail bed. It can be caused by local problems like periungual warts or fungus (onychomycosis), but the most common reason for separation is trauma. However, some systemic diseases can cause this like psoriasis and hyperthyroidism. Have your pediatrician look into these possibilities.
Spoon shaped nails. Spooned shaped nails are associated with anemia. Not sure if he has had blood work done but maybe the pediatrician can have blood work done to rule this out. Also make sure his shoes are not too tight as this could also cause detachment of the nail.
Trauma. Can be from shoes - take all his shoes with you and see derm or podiatrist.
Toenail. Technical term is onycholysis and usually falls off or lifts from nail bed due to trauma. Spoon shape could be a systemic problem like anemia.

How do I remove fungus in feet and toe nails?

Oral antifungal. There are many different options including topicals, oral antifungals, and lasers. The most effective treatments are oral antifungals such as sporanox, lamisil, (terbinafine) Diflucan and can have an 80-90% efficacy rate after 3 months. Topical creams have a much lower cure rate (40%) and have to be applied for 9 months. Lasers are also effective but there hasn't been a great amount of research in this area.
Several things. Try using topical antifungal if this does not work see a podiatrist.

Pure white toe nails w/ greenish yellow tint towards the top. I keep my toes clean & feet & don't let fungus build. I have lupus & Parkinson's? Issue?

White toe nail. The blood vessels under your nail in the nail bed is what gives your nails the pink color. The common cause of white nail is fungus or separation of the nail from the nail bed. Green/yellow discoloration is usually sign of an infection. I suggest you see a Podiatrist to have a fungal and bacterial culture done on the nail sample as well as to evaluate the nail to see if it is separated from bed.