Eynar Oxartum slɐoƃ sıɥ ɥɔɐǝɹ oʇ ʇsnɾ uʍop ǝpısdn sı
Today there were 24 km (almost 15 miles) in my training program.
I had a terrible sore throat, a runny nose and my tongue was aching. My first instinct was to put it off for another day. But Marathon Day will be in only 38 days, I cannot miss a single training day! I know I will run it willy-nilly, so the problem is not missing the marathon. The real problem is to run it undertrained, and get injured (perhaps even permanently injured).
So I followed the neck rule: if everything is fine from the neck downwards, and you feel sick only from the neck upwards, then you can run (I suppose an important exclusion it headache caused by running: if you start having a headache while running you should stop, but I am not an expert here). But you need to listen to your body, and if you start getting nausea or bad in any way, you have to be prepared to stop running.
The result was an outstanding long run. Nothing hurt while I was running (it seems it is a side effect of the body-produced epinephrine), and I witnessed one of the nicest sunrises ever at km 12.
[Please, note I am not a physician, I am just referring my experience. If you are looking for information about when to run while sick, you should not look for it by browsing the web]