I am so sorry for the loss of your friends. I cannot imagine how I would feel if one of mine died. One got lost and my heart still aches for him so much. I still cannot through his toys away. I don't let anyone else play with them just in case. I have two parrotlets who are pretty healthy per the vet.
The first thing I do when I get a new family member is take them to the vet immediately. They are quarantined for 30 days so no contact with any other bird in my home. I will wash my hands before and after handling the new bird. Once the vet gives us a thumbs up, they stay in quarantine for 30 days. No excuses and no exceptions.
No one gets to touch the new family member without washing hands. We wash our hands constantly as we also have chickens that are outside. So I have a large rain suit that covers me down to my angles and a hood that ties so I don't get anything on my clothes when I touch them. I also wear disposable gloves when I handle them and their eggs and their nest. We take off all of our clothes in the garage and hang them then wash our hands before entering the house to even think of touching our birds. Salmonella can kills parrots instantly.
We also have kept all of our birds in the bird room where the temperature is at a constant 78-80 and humidity at 40%. All foods give are Roudybush crumble, fresh dark or purple veggies, and some fruit and little seed.
Because Parrotlets are so tiny, even something minor can affect them. My parrotlets are very heathy. I believe Diablo was feeling under the weather a little so I had him sit closer to the humidifier so he'd get some extra moisture for whatever was ailing him. Now he's back to his territorial self again being king of the birds' room.
It sounds like your birds were sick before you got them or they were exposed to something in the environment that was dangerous. You would be surprised, candles, air fresheners, perfume, etc. are all killers. Especially teflon!! That's very poisonous and is in almost everything!
) or the rest of your flock. 
