there are different nail disease such as nail fungi

no it doesn't transmit disease unless if there is some virus in nail

Nail clubbing commonly occurs in relation with a heart or lung disease. The symptom of the disease is enlarged fingernails. In sixty per cent of the cases, nail clubbing occurs without relation to any disease.

Bacteria!!

No, it is not recessive. Nail-patella syndrome is inherited in an autosomal dominant manner. This means that possession of only one copy of the defective gene is enough to cause disease. When a parent has nail-patella syndrome each of their children has a 50% chance to inherit the disease-causing mutation.

A new mutation causing nail-patella syndrome can also occur, causing disease in a person with no family history. This is called a sporadic occurrence and accounts for approximately 20% of cases of nail-patella syndrome. The children of a person with sporadic nail-patella syndrome are also at a 50% risk of developing signs of the disorder.