According to the World Health Organization, there are more than 12,000 different diseases and disease-related symptoms that it categorizes. Based on the more than 139,000 clinical studies currently being run worldwide, you'd think we'd have a pretty tight grasp on dealing with the world's most pressing diseases. The reality is that we're not even close, and the evolution of medicine is an ongoing process.

Two areas where this process is still in its infancy -- which can also mean big bucks for investors if they back the correct horse -- is in Alzheimer's and Parkinson's disease research.

Alzheimer's: Where we've been & where we're headed

Based on the 2012 report from the Alzheimer's Association, 5.4 million people in the U.S. have the disease, and it's now become the sixth-leading cause of death. Of the documented cases of demetia, Alzheimer's accounts for about 80% of those cases.

At the moment, our understanding of the disease is minimal at best. One of the few FDA-approved drugs, Amyvid by Eli Lilly & Co. (NYSE:LLY) , isn't even a treatment at all. It's merely a process-of-elimination test that determines whether beta-amyloid plague markers exist on the brain and allows physicians to determine if a patient is at risk of onset Alzheimer's based on the results. Other Alzheimer drugs are nearing the end of their patent exclusivity come with a long list of side effects.

Other high-profile treatment attempts failed to reach the mark in recent months. Bapineuzumab, an Alzheimer's drug co-developed by Pfizer Inc. (NYSE:PFE) and Johnson & Johnson (NYSE:JNJ), failed in multiple late-stage studies and was eventually shelved. Eli Lilly's solanezumab shared a similar fate, but at least received a partial victory when deeper research noted marginal improvement in patients with mild forms of the disease. Solanezumab is one of three compounds taking part in a long-term international Alzheimer's study.

Given that we've seen seven Alzheimer pipeline drugs die in late-stage trials, the next most-promising one on the docket is Baxter International Inc. (NYSE:BAX)'s IV-administered Gammagard. In mid-stage trials, for those that received the optimal dosing of the drug, Gammagard held the disease at bay for four years. Those patients on the placebo or non-optimal dosing demonstrated a worsening of the disease. The upside is that in optimal dosing, this worked wonders. The downside: This was just a patient pool of 16 people. Late-stage results are due out on Gammagard in the first half of 2013.

As FierceBiotech points out, privately held Lundbeck and EnVivo Pharmaceuticals also have two late-stage Alzheimer's candidates that have shown statistical non-inferiority in trials. However, neither lacks the funding to move their drugs forward and, with a history of failure piling up, it could be difficult to find a partner.

Having spent $200 billion on Alzheimer's care last year, this is clearly a research area that's bound to merit big investment dollars. The question is, can anyone successfully penetrate the blood-brain barrier without a laundry list of side effects?