Mysql merge cuenta columna múltiple

Puedes usar union all para anular la dinámica de su conjunto de datos y luego agregarlo:

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