This is bacteria/fungus. From the photo it seems you have a lot of plants close together. Chances are one developed bacteria and then the splashing from watering that orchid allowed the bacteria to spread to other orchids in the group.
If it is really bad, then a systemic insecticide & fungicide once a week for 3 weeks then once a month thereafter for maintenance. If its not too bad then spraying them once a month will work fine.
It looks like fungus. I would recommend using a systemic fungicide on regular basis for a few months. I use one every three weeks. Also get the plants into an area where they get good light and plenty of air movement. Don't crowd them together.
If sections of the leaf are looking soggy and mushy to the touch I usually cut down to the section of the leaf that looks healthy and feels firm. Then I just dab on some cinnamon to the "open wound" to prevent any infection.