What if it is unclear whether a loss will be sustained? For example, it may be unclear whether pests will seriously damage a field. If the concern is great enough that people would normally cut short their vacation to deal with it, then melakha may be done on Ḥol Ha-mo’ed to prevent the loss (BHL 537:1 s.v. “davar”). If it is not recognizable that the melakha is being undertaken to avoid a loss, then, if possible, it should be done discreetly, so that people will not draw incorrect inferences about when it is permissible to work (MK 12b; SA 538:2).
One who is doing melakha to prevent a loss should do it in the easiest way possible. If it is possible to prevent the loss through a simple melakha, he should not do something more complicated (Rema 537:1). Therefore, if an item of clothing became stained, and the stain will set if the item is not washed, it is better to wash it in a washing machine rather than by hand.
Spiritual loss is also considered a loss. For example, if one comes up with a novel Torah interpretation and is afraid that if he does not write it down he will forget it, he may write it down. If he knows how to use a computer, he should type it instead, as this is less of a bother than writing.
When it is permissible to work in order to prevent a loss, it is also permissible to hire workers to do the necessary work. If possible, it is preferable to hire Jewish workers who do not have enough money to buy food for the festival. If none are available, it is preferable to hire non-Jews. If this is not possible either, the business owner should do the work. If he is unable to do so, he may hire Jewish workers even if they are not needy. Since it is unfair to take away their Ḥol Ha-mo’ed rest without pay, he must pay them for their work (Rema 542:1; MB ad loc. 5; SHT ad loc. 8).
If a water pipe in a wall bursts and is causing damage, a workman may be hired to fix the pipe, because this qualifies as davar ha-aved. However, he may not be asked to close the wall back up and paint it, because doing so on Ḥol Ha-mo’ed would not prevent any loss. Nevertheless, if the cost of the work will go up substantially if the worker needs to come back to finish the job, he may finish the job even on Ḥol Ha-mo’ed. Someone wealthy who can easily afford the additional expense may not rely on this leniency (see SHT 537:49, based on Ritva; see SSK 67:12).