I don’t believe this is true… in my experience they’re not any more prone to viruses than any other birds. They ARE slightly more prone to respiratory issues, because they’re respiratory systems are so much more sensitive than ours, but it’s not like, a thing that is super common. I’ve never had either of my tiels get sick, nor have I ever seen a tiel at the store I worked at get sick.
And most illnesses are not communicable between birds and humans. Some are, of course (psittacosis, for one, which can be serious and potentially fatal (but in the way flu can be fatal, like to old, very young, and immuno-compromised people)), but they’re pretty rare.
The respiratory illness you hear most about with parrots is aspergillosis, which CAN be serious for birds, and some birds are more prone to it (though i don’t think tiels are on that list; they can get it, of course, but I know that greys and I think amazons are particularly prone to it for whatever reason). But it’s caused by mold spores, not a virus; it’s not contagious. Generally it can be avoided by simple hygiene (and avoiding certain foods that are prone to carrying it).
What I would worry about, however, is the dust. They are extremely dusty and make the whole room dusty, so this could be something that could really aggravate already existing respiratory issues in people.
However you could still make it work. Obviously it depends on how bad your asthma is, but there are a lot of things you can do to control the dust situation. I have both asthma and allergies and I have two tiels who live in my bedroom (but my asthma isn’t very bad (mostly allergy triggered), and I’m not allergic to birds, so I can get away with cheating a bit). I would definitely invest in an air purifier, and don’t keep the bird where you sleep (do as I say, not as I do…). Also, a daily spray bath will help a lot with the dust. If you had one before, you may have an idea already if and how much they affected your asthma.
There’s also, of course, the option of looking into other birds. Cockatiels and birds in the cockatoo family are actually pretty unique in being dusty (though African Greys are, too), so another bird might not be nearly as much of an issue for you health-wise (I would list examples, but it would literally be pretty much everything that’s not in the cockatoo family or a grey; budgies, conures, parrotlets, lovebirds, lineolated parakeets, etc. etc. etc.).
Thank you so much for answering! ♡ My asthma has worsened a little over the years, so that’s why I am worried. It is mainly allergy-triggered but sometimes I have asthma out of nowhere and it has affected my relationship with my dogs and with my bunny (RIP) but it’s been so long since I’ve had a ‘tiel that I have no clue how I will react to them now. And tge thing that the person said to me had me worried even more because she said the diseases could kill me. O.o But cockatiels are my favorite pet bird because they are the only species of bird I’ve had, and I love their cutie faces. And my ‘tiel had and still has a very special place in my heart. And I really want one in the future because I live in a small house and I want to have a big space for it. :) But I’m rambling now, thanks!!