Can cockateils get diseases from being outside?

My 6 year old neighbor just gave me histwo cockatiels that they kept outside almost all the time and rarely cleaned. The cage was in pretty aweful condition. I am falling in love with my new birdies and now Im wondering if they are healthy. One is very cheerful and sings a lot, but the other seems to be much quieter and rarely sings. It also has a bald spot n its head when I got it so I dont know if thats a bad sign or the other bird just plucked its feathers or something. I really dont know much about birds so any helpwould be appreciated. Can bids get diseases from being kept outside?