See Rare Diseases by First Letter (B)
- B Cell Prolymphocytic Leukemia
- B-cell Lymphomas
- Babesiosis
- Bacterial Meningitis
- Balo Disease
- Balo's Concentric Sclerosis
- Bardet-Biedl Syndrome
- Bare Lymphocyte Syndrome
- Bare Lymphocyte Syndrome 2
- Barraquer-Simons Syndrome
- Barth Syndrome
- Bartter Syndrome
- Basal Cell Carcinoma, Multiple
- Basilar Migraine
- Batten Disease
- Becker Muscular Dystrophy
- Becker's Nevus
- Behcet's Disease
- Bell's Palsy
- Benign Paroxysmal Positional Vertigo
- Berger Disease
- Beriberi
- Berylliosis
- Best Vitelliform Macular Dystrophy
- Beta-galactosidase-1 Deficiency
- Beta-thalassemia
- Bethlem Myopathy
- Bile Duct Cancer
- Biliary Tract Cancer
- Biotinidase Deficiency
- Birdshot Chorioretinopathy
- Birt-Hogg-Dube Syndrome
- BK-virus Nephropathy
- Blastic Plasmacytoid Dendritic Cell
- Blastoma
- Blastomycosis
- Blepharospasm
- Bloom Syndrome
- Bone Cancer
- Botulism
- Bourneville Syndrome
- Bowen's Disease
- Brain Stem Glioma, Childhood
- Brain Tumor, Adult
- Brain Tumor, Childhood
- Breast Cancer, Male
- Brenner Tumor of Ovary
- Brittle Cornea Syndrome
- Bronchiolitis Obliterans
- Bronchiolitis Obliterans Organizing Pneumonia
- Bronchopulmonary Dysplasia
- Brown-Sequard Syndrome
- Brucellosis
- Brugada Syndrome
- Budd-Chiari Syndrome
- Buerger Disease
- Bullous Pemphigoid
- Burkitt Lymphoma
- Burnett Schwartz Berberian Syndrome
- Buruli Ulcer