Mysql fusion compte plusieurs colonnes

Vous pouvez utiliser union all pour annuler le pivot de votre jeu de données, puis agrégation :

select disease, count(*) total
from (
    select disease from mytable
    union all select additional_disease1 from mytable
    union all select additional_disease2 from mytable
    union all select additional_disease3 from mytable
    union all select additional_disease4 from mytable
) t
group by disease
order by total desc, disease