It's a scary world out there for you and your brain. Your grey matter is under assault from a host of almost bizarre attackers, from primitive protozoans to byproducts of living in the industrial age. Remember: if you're reading this, it's not too late. Once you begin to show signs of neurodegenerative disease, significant damage has already been done.
Below are a few of the nastiest and most toxic of the neuron and brain cell killers - the ones that never make the paper but likely are affecting hundreds of millions of us around the globe - that you should be aware of.
NOTE: One sign of neurodegenerative disease may be your dreams. Yes, how you dream may actually be the sound of your neurons screaming. Males, in particular, are vulnerable to certain neurological disorders that can display literally decades ahead of time in violent dreams. 
1. Drinking Water and Copper. Yes, isn't that nice? We build our homes with non-corrosive copper pipes, not realizinng that this mineral, in excess, has been found in several studies to significantly increase the risk of Parkinson's Disease.  Of course, Parkinson's is the disease where the dopamine-producing cells in the brain are killed, eventually leading to the incapcitating tremors and shaking associated with this disorder. Certain pesticides are linked with Parkinson's as well, but the strongest associated seem to be copper (and manganese). Copper actually misfolds a key protein that accelerates certain misfolded proteins in the brain associated with Parkinson's.
NOTE: The aluminum in water has also been associated with Alzheimer's. See my link on Aluminum and the Brain for more details.
2. Toxoplasma Gondii. Imagine a protozoan that makes its way up into your brain and lodges in your neurons, causing extensive damage (in some people), including increased risk for brain cancer and Parkinson's. Well, imagine no longer. This critter lives in about 15 percent of all of us and recent reserch shows it is a brain-changer. It is very hard to eradicate, even if you know you have it. It has been show to affect mood and even lead to traffic accidents.  One way that it injures the brain is by increasing dopamine levels.
CAUTION: Domesticed cats and uncooked foods are considered the most common way to get this pathogen.
3. Pesticides Of course, fruits and vegetables are very brain-protective. Unfortunately, many pesticides are not. Ziram, for example, has been linked to elevated Parkinson's risk.  See my link on Pesticides and Parkinson's for other examples. So far, most of the risk seems to be tied to actually working with the chemicals occupational However, there is some evidence that simple consumption in some cases may increase risk. The bottom line: go organic if you can afford it.
4. Iron. One rule for guys: "pump iron but don't eat it." First, iron was found to increase risk for heart disease.  And now it likely increases the risk for Parkinson's Disease.  Why is iron so male-toxic? It increases mitochondrial oxidative (free radical) damage.
5. Whey and Diet Drinks. Trying to be healthy with diet drinks? Lifting weights and consuming whey powder/drinks to put on muscle? If so, you're flooding your system with Excitotoxins. Read here about The Dangers of Excitotoxins. There is an additional risk, by the way, with diet drinks: about ten percent of the aspartame breaks down into methanol, a known neurotoxin. 
So how do you protect yourself? First of all, almost anything that protects your erections, your cardiovascular system and even your skin, will very likely protect your brain. You need to bathe your brain in abundant nutrients from a balanced, whole foods diet and push those nutrients upstairs with exercise. Cortisol is a proven brain killer, so managing stress is also critical. Many foods, such as blueberries, have superpowers to protect the brain. Read my link on The Brain for more information.
1) http://www.sciencenews.org/view/generic/id/61774/title/ Violent_dreams_may_predict_illness_in_advance
3) Lancet, 1987 Aug 1, 2(8553):238-41, "Raised cerebrospinal-fluid copper concentration in Parkinson's disease"
4) JOURNAL OF MOLECULAR NEUROSCIENCE, 28(2):125-141, Coenzyme Q10 provides neuroprotection in iron-induced apoptosis in dopaminergic neurons"
5) JAMA, 2007, 297(6):639-641, "The Iron-Heart Hypothesis"
6) Arch Intern Med. 1999, 159:1542-1548, "Iron, Atherosclerosis, and Ischemic Heart Disease"
8) Biology Letters, Published online before print July 27, 2011, "Incidence of adult brain cancers is higher in countries where the protozoan parasite Toxoplasma gondii is common"
9) Neurosci Lett, 2010 May 21, 475(3):129-31. Epub 2010 Mar 27, "The probable relation between Toxoplasma gondii and Parkinson's disease"
10) BMC Infect Dis, 2009 May 26;9:72, "Increased incidence of traffic accidents in Toxoplasma-infected military drivers and protective effect RhD molecule revealed by a large-scale prospective cohort study"