1. Don't panic: If you have been consistently training, don't worry, a couple days off will not cause you to loose fitness. Training breaks your body down. If you are sick and you go out and do a hard workout, you may do more harm then good. If you are sick and need time off, take the time off and don't rush yourself back. Be patient and smart. Think of it this way, if you need to take 2 days off to get healthy, but you don't take the time off, it will take you longer to get health. That 2 days may become 4 days. During that time you are doing more damage to your body, risking injury and it will take you longer to get back to normal training.
2. Be patient: The amount of time that you missed should be, at a minimum, equal to the time that was missed. If you are out for 2 days, then take 2 easier days to get yourself back into training. From there you can see how you are feeling. If it takes an extra day, then take the extra day. Better to be healthy and your body ready before putting heavy demand back on to it. Keeping the first point in mind, would you rather miss 2 days, take 2 days to build back into training, and be back to full training on day 5, or be sick for 4 days with crappy training, risk injury, and then only be back to full training in 8 days. Training is all about optimizing your recovery so you can push your body again as soon as possible. If you get sick you need to do the same.