There is no single agreed worldwide count of diseases. NIH estimates put rare diseases alone at more than 10,000, while liver organizations describe more than 100 types of liver disease. Neither figure is a count of every human disease.
Some questions have a more definite answer: there are five main hepatitis viruses. Others need a named classification system. A fatty-liver grade, a fibrosis stage, and a liver cancer stage measure different things.
How many diseases are there?
The total depends on how diseases and their subtypes are defined. Even rare-disease counts differ between major organizations because their definitions, source catalogues, and treatment of subtypes differ.[1][2][3]
| Question | Supported answer | What the number describes |
|---|---|---|
| How many rare diseases are there? | More than 10,000 in NIH estimates | Estimated distinct rare diseases; no universally agreed total |
| How many neurodegenerative diseases are there? | No worldwide total established by the sources reviewed here | A subset of neurological disorders |
| How many heart diseases are there? | No exhaustive total in the cited public-health classifications | Broad groups contain many individual conditions |
| How many liver diseases are there? | More than 100 types | An overview estimate that includes groups of subtypes |
| How many types of viral hepatitis are there? | Five main types: A, B, C, D, and E | Diseases caused by five different hepatitis viruses |
Sources: NIH, the NHS, the American Liver Foundation, and WHO. The rows describe overlapping groups and cannot be added together.[1][6][8][11][12]
How many rare diseases are there in the world?
NIH's Genetic and Rare Diseases Information Center estimates more than 10,000 rare diseases. The European Commission gives a range of 6,000–8,000. The two estimates come from different approaches to defining and counting diseases.[1][2][3]
In a 2020 analysis, Haendel and colleagues combined disease catalogues using the Mondo disease ontology. They counted 10,393 of the most specific disease terms, excluding broader parent categories. The authors described the result as preliminary and called for more rigorous analysis.[2]
The study reported 6,370 terms in at least three resources and 4,023 unique to one resource. This historical snapshot illustrates how much the selected catalogues can affect a count; it does not measure disease prevalence or discoveries since 2020.[2]
Rarity also depends on geography. GARD defines a rare disease as affecting fewer than 200,000 people in the United States. The European Commission's public-health definition uses fewer than five people per 10,000 in the EU.[1][3]
How many orphan diseases are there?
When “orphan diseases” refers to rare diseases, the same estimated count of more than 10,000 applies. FDA orphan-drug designations concern products intended for particular diseases or conditions, so counting designations would answer a different question.[4]
How many unknown diseases are there?
There is no measurable total of diseases that have not yet been identified. An undiagnosed patient may have an unusual presentation of a known disease or a previously unrecognized disorder. The NIH Undiagnosed Diseases Program investigates both, including through genome sequencing. A count of undiagnosed patients therefore cannot establish a count of unknown diseases.[5]
How many neurodegenerative diseases are there?
The sources reviewed here do not establish a single worldwide total of neurodegenerative diseases. NIH identifies Alzheimer's and Parkinson's as the most common examples, alongside conditions such as Huntington's disease and amyotrophic lateral sclerosis (ALS).[6]
The often-repeated figure of more than 600 refers to neurological disorders overall, including epilepsy and migraine. Its scope is broader than neurodegenerative disease.[7]
How many different types of heart disease are there?
There is no fixed, exhaustive total in the cited classifications. Heart disease includes disorders of the coronary arteries, heart muscle, valves, rhythm, and development, as well as heart failure. Each group contains different conditions, and several can occur together.[9][10]
For example, valve disease can be classified by the valve affected and by whether it is narrowed, leaks, or has another abnormality. Counting “valve disease” as one type gives a different answer from counting each specific valve disorder separately.[10]
The same counting problem applies to “heart conditions” and “heart problems.” A patient-information list groups conditions for explanation; its headings do not establish a worldwide total.
How many cardiovascular diseases are there?
The NHS lists four main cardiovascular disease groups: coronary heart disease, peripheral arterial disease, aortic disease, and stroke. Its page also states that there are many different types of cardiovascular disease, so four is a count of the groups in that overview.[8]
Cardiovascular disease covers the heart and blood vessels. It therefore extends beyond diseases of the heart itself. A list focused on artery disease and stroke also has a different scope from one organized around heart valves, rhythm, and muscle.[8][9]
How many liver diseases are there?
There are more than 100 types of liver disease, according to the American Liver Foundation's 2025 fact sheet. Many fall into groups of subtypes, so the figure is an overview estimate.[11]
The causes include viral infections, metabolic disease, alcohol, autoimmune disease, inherited conditions, medicines and toxins, and cancer. Cirrhosis describes advanced scarring that can result from several of these causes.[11]
How many types of hepatitis are there?
There are five main types of viral hepatitis: A, B, C, D, and E. WHO sometimes calls these five “strains,” but they refer to different hepatitis viruses. A count of genotypes or variants within one virus is a separate question.[12]
Hepatitis means liver inflammation. It can also have nonviral causes, including autoimmune disease, alcohol, and medicines. Five therefore answers the question about the main hepatitis viruses, rather than every possible form of hepatitis.[11][12]
How many types of cirrhosis are there?
Cirrhosis has two broad clinical states: compensated and decompensated. It can also be described by its cause, such as alcohol-associated disease, chronic viral hepatitis, or metabolic liver disease. These are different ways of classifying the same condition, so there is no single count that covers every meaning of “type.”[11][17]
How many grades and stages of liver disease are there?
The answer depends on what is being assessed: fatty-liver grading commonly uses three abnormal grades; METAVIR fibrosis staging has five categories, F0–F4; and cirrhosis has two broad clinical states. The numbers cannot be substituted for one another.[13][16][17]
| Assessment | Number of categories | Labels | What it describes |
|---|---|---|---|
| Steatosis grading | Three abnormal grades, plus grade 0 | S0–S3 | Amount of liver fat |
| Simplified fatty-liver progression | Four broad states | Steatosis, steatohepatitis, fibrosis, cirrhosis | A possible course of disease |
| METAVIR fibrosis staging | Five, including no fibrosis | F0–F4 | Extent of scarring |
| Cirrhosis clinical state | Two | Compensated, decompensated | Whether major complications have developed |
| Child–Pugh classification | Three classes | A, B, C | Severity assessed from clinical findings and blood tests |
| Hepatitis C infection | Two clinical forms | Acute, chronic | Early or persistent infection |
| BCLC staging for hepatocellular carcinoma | Five stages | 0, A, B, C, D | Cancer burden, liver function, and patient status |
Sources: radiology guidance, NHS patient information, WHO, a hepatology review, CDC, and NCI. Each row uses its own definitions.[13][14][16][17][18][20]
How many grades of fatty liver are there?
Fatty liver is commonly graded as mild, moderate, or severe: grades 1, 2, and 3. Systems that include grade 0 have four categories overall, with grade 0 indicating absent steatosis. Reports may use the labels S0–S3.[13]
Ultrasound grading describes fat-related imaging findings. It does not by itself assign a fibrosis stage. Grade 3 fatty liver and F3 fibrosis are different findings.[13][16]
How many stages of fatty liver are there?
Patient guides often describe four broad stages: simple fatty liver, steatohepatitis, fibrosis, and cirrhosis. This simplified sequence describes possible progression. Many people with liver fat do not progress through all four.[14]
Steatohepatitis involves inflammation and liver-cell injury; fibrosis describes scarring; cirrhosis is advanced scarring that changes the liver's structure. These features can coexist.[14]
Older sources use NAFLD and NASH. The newer terminology uses metabolic dysfunction-associated steatotic liver disease (MASLD) and metabolic dysfunction-associated steatohepatitis (MASH). MASLD requires liver fat and at least one specified cardiometabolic risk factor; it does not cover every cause of a fatty liver.[15]
How many stages of liver fibrosis are there?
The METAVIR system has five stages, F0 through F4. F0 means no fibrosis, and F4 means cirrhosis. Other scoring systems have different scales, so a fibrosis-stage count needs the system's name.[16]
| METAVIR stage | Finding |
|---|---|
| F0 | No fibrosis |
| F1 | Fibrosis around portal areas, without fibrous bands called septa |
| F2 | Portal fibrosis with septa |
| F3 | Numerous septa, without cirrhosis |
| F4 | Cirrhosis |
Source: WHO's METAVIR reference table in its 2024 hepatitis B guidelines.[16]
How many stages of cirrhosis are there?
Cirrhosis has two main clinical stages: compensated and decompensated. Decompensation means that complications such as abdominal fluid accumulation, bleeding from enlarged veins, jaundice, or hepatic encephalopathy have developed.[17]
The Child–Pugh system separately groups severity into classes A, B, and C. F4 is a fibrosis designation for cirrhosis; it does not automatically mean decompensated cirrhosis. These classifications describe different aspects of liver disease.[16][17]
How many stages of hepatitis C are there?
Hepatitis C is described as acute or chronic. The early infection may clear, while persistent infection becomes chronic. CDC discusses spontaneous clearance within the first six months; liver scarring is assessed separately from the duration of infection.[18]
An F0–F4 result describes fibrosis, not five stages of the hepatitis C virus. A person with chronic hepatitis C can have different degrees of liver damage.[16][18]
How many stages are there in liver failure?
There is no single numbered staging system covering every form of liver failure. The commonly quoted sequence of four stages describes chronic liver disease: inflammation, fibrosis, cirrhosis, and liver failure. In that sequence, liver failure is the last state.[11]
Acute liver failure can occur suddenly, without preceding years of fibrosis and cirrhosis. NIDDK distinguishes it from chronic liver disease when explaining why a transplant may be needed.[19]
How many stages of liver cancer are there?
For hepatocellular carcinoma, the most common primary liver cancer, BCLC has five stages: 0, A, B, C, and D. The AJCC/TNM system uses four main stage groups, I–IV, with subdivisions. Both answers need the staging system attached.[20]
BCLC considers the cancer, liver function, physical status, and cancer-related symptoms. A stage in BCLC cannot be read as the same number in TNM, a fibrosis score, or a fatty-liver grade.[20]
