LONDON, England (CNN) -- The 1992 Hollywood movie "Lorenzo's Oil," depicts the true story of Lorenzo, a five-year-old boy who suffered from adrenoleukodystrophy (ALD), a rare and incurable disease that slowly destroys the entire nervous system.



Michaela Damin and her son, Nick, who suffers from  an extremely rare disease that affects the heart.



The movie showed how Lorenzo's grave physical and mental decline was finally stopped when his tireless parents found a treatment based on a mixture of oils, despite skepticism from doctors.



The film illustrated perfectly the struggle faced by patients suffering from any of 6,000 known rare conditions worldwide, generally known as orphan diseases.



Michaela Damin, founder of the Barth Syndrome Trust, whose son, Nick, suffers from this crippling syndrome that affects the heart and immune system and only has 130 known sufferers worldwide, told CNN: "It can take years to get the correct diagnosis and even then there may be no expert who knows what to do next."



The fact remains that rare diseases are still mostly overlooked by pharmaceutical companies because of the small patient population and lack of financial incentives.



Drug companies tend to focus on the more lucrative "blockbuster drugs" for common diseases, at the expense of cures for rare conditions.



The term "orphan drug" was originally coined because the pharmaceutical industry took little or no interest in discovery, development and marketing of drugs for rare diseases.



According to the European Organization for Rare Diseases, rare diseases affect between three and five percent of the population in developed countries.



In the European Union, a disease is considered rare if it affects fewer than five in 10,000 people, while in the United States the disease must affect fewer than 200,000 people.



Many countries, such as the United States, Japan and the United Kingdom, have therefore attempted to encourage pharmaceutical companies to invest in orphan diseases by giving them tax incentives, fee reductions and extensions of patent rights.



The United States was the first country to develop an orphan drug law in 1983, followed by Japan and Australia. In 1997, the Australian Orphan Drugs policy was set up to help manufacturers overcome the high cost of marketing drugs which have not proved to be commercially viable.



Since 1998, the EU has also devoted time and resources to rare diseases and passing laws that have led to more research and development in the field.



But even when treatments are developed, they generally remain extremely expensive.  See where to get help for rare diseases »



Genzyme, one of the pharmaceutical companies that focuses primarily on rare diseases, has long charged more than $300,000 a year for patients on Cerezyme, a drug used to treat Gaucher disease, an extremely rare condition that causes bone deterioration, among other symptoms.



Do you know someone who has a rare disease? Tell us in the Sound off below or send us an iReport



Only about 5,000 people around the world are taking Cerezyme. Genzyme says that justifies the high price.  See images of children living with rare diseases »



"The cost of developing therapies for very rare diseases is not substantially lower than those for common diseases and can take years," Steve Bates of Genzyme told CNN.



In their 2005 book on orphan drugs, Daniel Hagn and Oliver Schoffski wrote that on average, only one of 5,000 to 10,000 substances tested gets as far as market authorization and that it takes eight to 12 years and €895 million ($1.2 billion)  to develop and market a new drug.



Bates said Genzyme needs to recoup those high costs and that profits are the motive for further research: "Our profits are then re-directed into research for new drugs."



Genzyme's global revenue in 2008 was $4.6 billion, Bates told CNN.



A spokeswoman for French drug firm Hoffmann-La Roche told CNN: "There is a Roche Working Group on orphan diseases, but not a separate department." She declined to say how much the company's resources were allocated for research into the diseases.



While Pfizer spokesman Oliver Stohlmann told CNN in a written statement
what is the number of rare diseases worldwide
6,000
question: what is the number of rare diseases worldwide, answer: 6,000