Here I copied this for you from SB, TSandM stated:
"With a viral respiratory infection, there are really several issues that can pertain to diving.
First is what we all know well, which is the ability to equalize ears and sinuses. The ability to clear ears is pretty easy to test on land, and if you can't equalize on land, or only with significant difficulty, it's probably not a good idea to get in the water. Sinus congestion is more difficult to assess and probably more something each individual has to learn about himself. Mild congestion may be fine, severe plugging is obviously not, but there's a lot of middle ground where you have to decide for yourself whether it's worth the risk of a block.
The second issue is lung congestion. This can involve mucous plugging of the airways, swelling of the lining of the airways, and bronchoconstriction (which is the phenomenon causing much of the problem with asthma). As spectrum said above, the big danger is air trapping and arterial gas embolism. If one is coughing up large amounts of sputum, or has a significant wheezing component to the cough, then it's definitely time to stay out of the water. Toward the end of the illness, when the cough is mild and dry, it's more likely to be safe, but one still has to recognize whether there is a bronchospastic component to the cough. Some of us have what's known as "reactive airways disease", which is an asthma-like process associated with viral illnesses. Although not true asthma, it has the same risks for a diver.
The third problem is the potential effects on the inner ear. Respiratory viruses can involve the semicircular canals, which determine your balance. Any sense of wooziness or instability on your feet could translate into vertigo in the water.
Hope that's useful information."
