I am very sorry to hear about your parakeets. Unless these were new birds that you only had for a few days or weeks, I would not suspect a virus as the cause of death. While a viral infection is a very common cause of sudden death in pet birds, it normally occurs when a new bird that is a carrier is brought into the home or extreme stress causes a latent virus to emerge.
In your case, I would first consider environmental causes before you bring new birds into the home. Common household hazards which can cause sudden death are fumes from non-stick cookware used at high temperatures, cleaning solutions, air fresheners or carbon monoxide from an improperly working heater. Seasonal concerns can be scented candles or other scented items, or consuming poisonous plants such as poinsettias. Again, before you consider getting another bird, please do a search for common household hazards, toxins and poisons for pet birds. This list I provided is only a few common hazards. And if this happened when a heater had been turned on for the first time in a while, I would highly recommend a carbon monoxide detector for your family’s safety. A small amount can kill a bird, but might not be strong enough to harm a person other than cause some vague symptoms.
If you have ruled out poison of any type, then you can prepare for new birds. The cage should be cleaned in a bleach solution or you can search for a bird safe disinfectant. Again, please do a search for recommended dilutions and disinfectants. All wood and porous materials need to be thrown away and replaced. Smaller items made of plastic & metals can be thoroughly washed and then put in the top of your dishwasher for a regular cycle – this will sanitize them.
I would not recommend bringing home new birds until after the holidays, when you have resumed your normal, daily schedule. A new bird needs time to settle in and this is not a good time for this since most people will have a lot of visitors and household activity related to the holidays or possibly be gone more than usual.
I hope you can determine a possible cause of death so that you can feel safe in bringing in a new pet bird or birds.