Marcos: Bom dia, doutor. Estou doente.
The doctor is treated with "doutor" ("doutora" would be the feminine). Sometimes with the name, but always with "doutor". ("Doutor Souza").
The regular verb "doer" (to feel pain or hurt), is defective. It just exists in two conjugations. Singular (dói) and plural (doem).
Now, it is time to learn some new words:
"Trata com que frequencia" / Treat it with what frequency
"Eu faço-o três vezes por semana" / I do it three times a week
"Tenho/Tens (o que)" / I/You have got (what)
The disease (prolonged sickness) is determined with the verb to have "ter" / to have,