Why does my dog gets a on and off red eye on his left?

My dog is 4 years old, a couple of weeks ago we noticed that his left eye is reddened with gummy secretions but we assumed that its just some sort of irritation caused by dust. After a day, his left eye turned back to its normal color then another day passed it return to red. This has been happening to him for almost a month now.Correct me if I'm wrong, but from what I know it isn't an infection because the other eye should have been infected if it is.

take him to the veta red film must be covering his eye

Your pup needs to see the vet. Your description is some sort of infection, and yes, your pup can get an infection in only one eye. The common theory that both eyes should be involved is based on human behavior - we use our hands to transfer infections from one eye to the other. Dogs don't do that. Why it is coming and going is another issue. Do you do anything to the reddened eye? Flush it? Wipe it clean? These things would reduce the number of bacteria, and possibly make the infection less visible until the bacteria multiply back to the previous level.A simple ocular antibiotic will help you to rid the infection - but the vet has to see it before he/she can prescribe an antibiotic.Good Luck, and Happy New Year