Maggio, 32, suffered breathing difficulties after Napoli's UEFA Europa League loss to Porto in March, and consequently underwent emergency surgery after arriving back in Italy.
The wing-back said he had previously experienced problems in training, before it was revealed he had a collapsed lung.
"The problem had started in Naples, as I felt pain after a knock to the chest in training. I went to Oporto and on the way back felt ill," Maggio told +N.
"I couldn't breathe and asked to go to hospital. From the initial test they could already tell my lung was no longer there.
"It was a real psychological blow, going from feeling perfectly fine to lying on a hospital bed."
Maggio said he encountered a new experience in his return against Sampdoria in the Serie A on Sunday, as he did not expect to feel so nervous.
"I did feel something. I could've thought about dealing with any kind of injury, but in all honesty I didn’t prepare for this one," he said.
"Then I got the all-clear from the medics and my first thought against Sampdoria was psychological, but it all passed."
Maggio is World Cup-bound after being selected in Cesare Prandelli's initial squad for Brazil, although he said he would understand if he missed out on the final squad due to his latest health setback.
"I hope to be there. Now we have three or four days to train together and see where we are. At the moment the important thing is to be in the preliminary list, but I put myself in Prandelli's shoes and realise there can be some worries about having a player who was out for a while," Maggio conceded.
"Lorenzo Insigne is there too and it's obvious I'd be happy if he went to the World Cup. He is still young and when he gets pig-headed I always remind him that he has a bright future ahead."
Maggio said hosts Brazil could be found out at the World Cup, while holders Spain remain the team to beat.
"The main team to worry about at the World Cup is Spain," he said.
"We faced Brazil in the Confederations Cup last season and to be honest they didn't impress me very much, even if we were in great shape.
"Anything can happen at the World Cup, as Argentina are in the running, while Colombia and Belgium can surprise."
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