Porte told Australian selector Bradley McGee that he would be in no condition to race the 254km World's course in a conversation last week, and elaborating in an interview with Cycling Central, Porte said he'd not been totally healthy since January.
"I haven't had the best of health this year," said Porte. "I've been in Manchester the last week speaking with the doctors and we've found that during the Tour I had a mild case of pneumonia.
"I'm not going to beat myself up too much, I had a year like this three seasons ago, and I've had some good ones since.
"You have to have a bad year every now and then, just to put it all in perspective, but to be honest I'm pretty happy to draw a line through the year and move on."
Though Porte has enjoyed spells of good form this year, he said he's been unable to properly shake a string of chest infections from his system the first of which he contracted back in March. Porte has been in consultation with a number of medical experts both within the team and in the United Kingdom to diagnose the problem, and hopes to be in better shape next year.
"For me it's the end of the year now, and hopefully I can have a better season next year."
Porte first fell ill at Tirreno-Adriatico, forcing him to withdraw midway through the race. After failing to recover the ground he'd lost in form, and lost racing, he made the decision in April to withdraw from the Giro d'Italia, his key objective of the season.
The Australian began to turn things around toward the end of May and was reportedly in good health again at the team training camp in Tenerife, helping him produce a solid showing at the Criterium du Dauphine and indeed the Tour de France.
But his chest problems arose once more after the Vosges mountains, ending his aspirations of a high finish in Paris.
Porte hasn't finished a race since the Tour, abandoning in Hamburg at the Vatenfall, and the following week at the Grand Prix Plouay.
With his decision to pull himself out of the UCI Road World Championships, Porte has also closed his season for 2014.
SBS will broadcast the 2014 UCI Road World Championships live.
The first five days of competition will be live-streamed on the Cycling Central website, while the final two days will be broadcast on SBS ONE and streamed online. All times listed are AEST.