You will find plenty of people on Usenet servers who are genuine experts in their field. You will also find plenty who are not and, unfortunately, it is on you to decide what advice is good and what advice should be disregarded wholesale. A bit of common sense will go a long way here.
One characteristic you might notice about the posts made by health professionals is that they will always advise you to go to your doctor for any serious condition and they will always tell you that they cannot diagnose online. This is just basic science. A medical condition can only be diagnosed by someone with physical access to you. Even when a doctor does make a diagnosis, they are going with the most likely cause of your ailment. Doctors sometimes get it wrong, even in person.
If you have health issues that indicate something serious, you cannot use the Usenet to seek advice, you have to go to a doctor first. You can, however, make great use of the Usenet for information about how to live with your condition, if you have one, after a doctor has made sure that there are no emergent conditions. If you are having a symptom such as pain in your upper left chest, you need to go to a doctor. If you have a stiff arm or problems with your back, you very well may want to ask about it on Usenet, as these types of symptoms rarely indicate anything life-threatening.
Filter the advice you get with some common sense. Some people will recommend very out-there alternative therapies that may or may not be worth anything. In some cases, their posts will be full of pseudo-scientific drivel that was designed to make themselves sound knowledgeable. The great thing about Usenet, however, is that someone who actually knows what they are talking about will usually chime in and give you sensible advice. There is no shortage of scientists - medical or otherwise - on Usenet, which does make it a bit more trustworthy than standard Internet forums where getting reliable advice is concerned.
There are newsgroups for people with certain conditions and you may want to give them a try. These are good places to get advice about living in general. If you have diabetes, for example, you will likely find plenty of groups where other diabetics talk about how they get by and how they keep their lives rich and enjoyable. These can be great sources of support.
The Usenet is a great source of information, but you have to have some critical thinking skills to find the best of what it has to offer. Your chances of finding someone qualified, however, are relatively good on this service.