Understanding Rare Diseases: What They Are, What Causes Them, and Why It Matters
4 min read
If you or a loved one has been diagnosed with a rare condition, you’re not alone, and you deserve clear, honest information to help you navigate the journey ahead.
The word “rare” can feel isolating. But here’s something important to know: rare diseases are, collectively, far more common than most people realise. Taken together, they touch hundreds of millions of lives around the world. More than 300 million people around the world have one of the more than 7,000 known rare diseases.1
What does "rare" actually mean?
A disease is called “rare” based on how many people have it, not how serious it is. Different countries draw the line differently, depending on their population size and healthcare systems. What they all share: these are conditions that don’t affect most people, which also means they’re often less well-understood, less funded, and harder to diagnose.
Some rare diseases are mild and manageable. Others require ongoing, specialised care throughout a person’s life. Most are long-lasting, and for the vast majority, there is still no approved cure or targeted treatment.
Being rare doesn’t mean being unimportant. It means the healthcare system needs to work harder to support people living with these conditions, and that’s exactly why awareness, research, and community matter so much.
This article will help you understand what rare diseases are, what may cause them, how they are grouped, and why genetics often plays an important role in understanding these conditions.
What causes rare diseases?
- Genetic changes: Around 70–80% of rare diseases are linked to changes in genes, the instructions that tell our bodies how to grow and function. These may be inherited from a parent or happen spontaneously. ³
- Metabolic disruptions: Some diseases occur when the body can’t properly break down or process certain substances, causing a build-up that disrupts normal bodily functions.
- Environmental or acquired factors: In some cases, exposure to certain toxins or infections can trigger a rare condition, sometimes in combination with an underlying genetic vulnerability.
- Unknown causes: For a significant number of rare diseases, the exact cause still isn’t known. But research is ongoing, and answers do emerge over time.
How are rare diseases grouped?
Doctors and researchers organise rare diseases in a few different ways. This helps with diagnosis, treatment, and research, and helps you understand where your (or your loved one’s) condition fits.
By cause
Most rare diseases (about 70–80%) have a genetic root. The rest involve the immune system, certain rare cancers, or causes that are not yet fully understood.
By the body system affected
Rare diseases can involve almost any part of the body:
- Brain & nervous system
- Metabolism
- Lungs & breathing
- Skin
- Blood & immune system
- Heart & circulation
- Bones & muscles
By when symptoms appear
- Congenital (present at birth)
- Childhood-onset (Approximately 50% of them are present in early life.)
- Adult-onset
Is it always inherited?
This is one of the most common questions families ask, and the answer is: not always. There’s an important distinction between genetic diseases and inherited ones.
Inherited rare diseases are caused by genetic mutations passed from one or both parents. Well-known examples include cystic fibrosis, sickle cell disease, and Duchenne muscular dystrophy. A family history may reveal similar conditions, but not always, especially if parents are unaffected “carriers.”
Non-inherited (sporadic) rare diseases happen on their own. They may be caused by:
- De novo mutations: Brand new genetic changes that occur for the first time in the affected person, not inherited from either parent, but seen in embryo. These are a major cause of rare neurodevelopmental conditions.
- Spontaneous mutations: These are small random errors that occur when cells copy DNA. Examples include Down syndrome and achondroplasia.
- Somatic Mutations: These are changes that occur in specific cells of the body, not passed to children. Many rare cancers fall into this category.
- Epigenetic Changes: alterations in how a gene works, without changing the DNA itself. These may be influenced by environmental or developmental factors, and are not necessarily passed down through generations. Example: Fragile X Syndrome.
- Environmental Factors: Some rare diseases are caused by a combination of minor genetic variations and environmental triggers, which makes the inheritance patterns less clear. Examples: Neurodegenerative diseases (like Parkinson’s) and congenital anomalies (heart diseases).
A person can be the only one in their entire family with a rare genetic disease. It simply means the mutation occurred for the first time in them.
Why does genetics matter so much?
Around 95% of rare diseases do not have an approved treatment plan. Understanding the genetic basis of a rare disease isn’t just about science, it has real, practical benefits for patients and families:
- Getting an accurate diagnosis: Genetic testing is helpful for getting a clear diagnosis, especially when symptoms are similar to those of other conditions.
- Choosing the right treatment: Choosing therapies that target specific genetic mutations. For instance, gene therapies and precision medicines are increasingly being developed for rare disorders.
- Predicting disease course: Understanding the genetic basis of a condition can help predict its progression and which organs may be affected.
- Family planning & recurrence risk: Genetic information helps families understand the risks of recurrence and consider options such as genetic counseling or prenatal testing.
- Advancing new therapies: It is beneficial for ongoing research because it helps develop targeted treatments and better disease models.
What does living with a rare disease look like?
Progress is being made, especially in precision medicine and “orphan drugs” (therapies developed specifically for rare conditions). But significant challenges remain:
- Long delays to diagnosis, because healthcare systems are built around common conditions
- Limited access to specialist centres and diagnostic tools
- High cost of treatments, which are often not widely available
- Too few trained specialists in rare disease care
- Social isolation and the psychological weight of living with a poorly understood condition
If you’re navigating a rare disease diagnosis, for yourself or someone you love. know that advocacy, community, and connecting with specialists matters. You shouldn’t have to figure this out alone.
References:
- https://link.springer.com/article/10.1007/s44162-026-00179-1#Tab2
- https://www.nature.com/articles/s41431-019-0508-0#Abs1
- https://health.ec.europa.eu/rare-diseases-and-european-reference-networks/rare-diseases_en
- https://www.thelancet.com/journals/langlo/article/PIIS2214-109X(24)00056-1/fulltext#:~:text=Around%2080%25%20of%20rare%20diseases,people%20living%20with%20rare%20diseases?
- https://pmc.ncbi.nlm.nih.gov/articles/PMC7229282/?
- https://pmc.ncbi.nlm.nih.gov/articles/PMC11881545/
- https://www.arensia-em.com/blog-15-everything-you-need-to-know-about-rare-diseases
- https://pmc.ncbi.nlm.nih.gov/articles/PMC11881545/
- https://www.ijcmph.com/index.php/ijcmph/article/view/12439
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