I must be the only one that Zpacks/Zithromax has no effect on. Or my GP prescribed them when it was something beyond it's capability. Twice and no response.
It usually works for me, but not every time. A guy at work was just on his 3rd antibiotics before getting results. Too many things going around and docs don't know what to give first.
There are some colds now that are getting resistant to antibiotics, and some people who take them too much build up a tolerance to antibiotics and they do not work...