Is mild interstitial thickening caused by intestinal lung disease? or can it be caused by something else instead like a cold?

Same thing.: Interstitial thickening is a finding, typically on an xray, which points to the diagnosis of interstitial lung disease. ILD is caused by any number of exposures (asbestos, nicotine, asthma exacerbations) which cause inflammation and scarring of the lungs. Once there, there is no way to reverse it, but you can work to prevent further inflammation by controlling asthma and avoiding other exposures.