Not cold, nor virus. Instead, leaf miners.
Here in the northwest, leaf miners are very common pests of leafy greens (spinach, beets, and chard). See "Leafminers" - http://ipm.ucanr.edu/PMG/r732300311.html. That article describes the life cycle and damage made by serpentine leafminers (narrow winding trails) whereas your spinach has blotch leafminers (large whitish irregular shapes.) Treatment is the same for both.
Leafminers are small flies which lay their eggs on the surfaces of the leaves. When the eggs hatch, the larvae (youngsters) burrow into the leaf in order to feed between the top and bottom layers. If you hold a damaged leaf up to the light, you may see the quite small, worm-like larvae lined up at the edge of the light-colored spots where they are feeding. When they are done feeding, they drop onto the soil.
Least toxic management includes regular search-and-destroy missions to get rid of the eggs which are laid in small groups of up to 4;. The eggs are white, elongated, and set side-by-side. Patrol the spinach daily in order to stop the damage as soon as possible.
At the same time, keep an eye out for heavily infested leaves; remove and discard them in the trash before the larvae drop to the ground where they’ll complete their life cycle. And whenever you see small whitish areas of new damage, stop the larvae in their tracks by squishing that area between two fingers.
To avoid the problem next year, plant your spinach (seeds or transplants) in a different garden bed and immediately cover the planting with row cover to exclude the adults. To do so, loosely blouse row cover over the planting, and then secure the edges. Row cover is a filmy white, non-woven fabric available in most garden centers but you can substitute bridal tulle. See “How to Install a Floating Row Cover” http://cru.cahe.wsu.edu/CEPublications/FS089E/FS089E.pdf