Scuba diving although not considered an extreme sport is still one that involves placing your body under pressure. It is recommended before you undertake a Scuba Diving Certification that you first have a full medical by your doctor.
There are certain medical conditions which will prevent you from diving.
1) Asthma - the level of your asthma will determine if you can or can't dive at all. You should make sure that your doctor advises you regarding this.
2) Heart Problem - the pressure placed on your organs as you descend is increased if you have a heart problem
3) Lung disease or disorder - which prevents you from breathing freely, again consult your doctor
4) Epilepsy or Neurological disorders - your doctor will definitely not clear you to dive if you suffer either of these.
5) Bipolar or Depression - again everyone and their illness are different, consult your doctor for your specific diagnosis.
There are also some conditions which will pass, but pose a health threat if you try to scuba dive whilst suffering from one of these.
6) Ear Infections - Due to the importance of equalizing your air cavities (i.e. your ears) as you descend on a scuba dive, you cannot dive with any type of ear infection. Thankfully with treatment and ear infection will pass and you will be able to dive again in the future.
7) Pregnancy - although only temporary, it is not advised for anyone who is pregnant to scuba dive. There have been many studies but there is no conclusive evidence one way or the other. Given that pregnancy is such a gift and lasts only 9 months, it would be a little silly to jeopardize that for a scuba dive.
8) Cold and Flu / Sinus trouble - if you are temporarily suffering from the flu simply don't dive, treat the Cold as you would normally and hope for a quick recovery so that you can get back to diving.
9) Joint Injury - Ensure any injuries to joints or muscles have healed before going in for a dive.
When you visit your doctor for your pre-dive medical, be sure to ask if any medications which you may be taking will effect you whilst Scuba Diving.
Obviously anyone who is under the influence of drugs or alcohol should not dive. Remember that you are not only risking your life but that of your dive buddy. As they may rely on you in the instance of a real emergency.
Remember that this is your life, you are only given one. We encourage you to get off your couch and into the water, but you must do this safely and responsibly.
NB. It is also important not to fly within 12 hours of your last dive, and to be safe I would suggest wait for 24 hours. Remember if you do have to miss out on a single dive due to a temporary medical condition, you can always opt for snorkeling.