In 2003, a group of scientists and executives from the National Institutes of Health, the Food and Drug Administration, the drug and medical-imaging industries, universities and nonprofit groups joined in a project that experts say had no precedent: a collaborative effort to find the biological markers that show the progression of Alzheimer’s disease in the human brain. Now, the effort is bearing fruit with a wealth of recent scientific papers on the early diagnosis of Alzheimer’s using methods like PET scans and tests of spinal fluid. More than 100 studies are under way to test drugs that might slow or stop the disease. And the collaboration is already serving as a model for similar efforts against Parkinson’s disease. A $40 million project to look for biomarkers for Parkinson’s, sponsored by the Michael J. Fox Foundation, plans to enroll 600 study subjects in the United States and Europe.