Nested Forms in Rails 4.0.3

Im struggling with a nested form in Rails 4, this is the first time that I made a form of that kind. I have read lots of documentation but Im not able to make it work. :-(

I have two models: Person and Disease. One person can have many diseases and one disease belongs to person. Looks quite simple. The form for Diseases is not saved in the database.

Person Model:

class Person < ActiveRecord::Base
  belongs_to :user
  has_many :diseases, :dependent => :destroy #if you delete a person you also delete all diseases related
  has_many :appointments, :dependent => :destroy
  validates_presence_of :name, :email
  validates :name, :length => {:maximum => 50, :too_long => "name is too long"}
  VALID_EMAIL_REGEX = /\A[\w+\-.]+@[a-z\d\-.]+\.[a-z]+\z/i
  validates :email, format: { :with => VALID_EMAIL_REGEX , message: "is invalid" }

  accepts_nested_attributes_for :diseases

end

Disease Model:

class Disease < ActiveRecord::Base
  belongs_to :person
  has_many :treatments
  validates_presence_of :name, :start
  validates :name, :length => {:maximum => 50, :too_long => "is too long, you can use the description field"}
  validate :start_must_be_before_end, :unless => [:chronical, :unfinished], :presence => true
  validates :end, :presence => true, :unless => [:chronical, :unfinished], :presence => true
  validates :description, :length => {:maximum => 5000, :too_long => "is too long"}

  def start_must_be_before_end
    if self[:end] < self[:start]
      errors.add(:start, "must be before end time")
      return false
    else
      return true
    end
  end

end

People Controller:

  def create
    current_user
    @person = Person.new(person_params)
    @person.user_id = @current_user.id

    respond_to do |format|
      if @person.save
        format.html { redirect_to @person, notice: 'Person was successfully created.' }
        format.json { render action: 'show', status: :created, location: @person }
      else
        format.html { render action: 'new' }
        format.json { render json: @person.errors, status: :unprocessable_entity }
      end
    end
  end

def person_params
  params.require(:person).permit(:name, :surname, :gender, :birthdate, :bloodtype, :user_id, :phone, :email, diseases_attributes: [:id, :description] )
end

Form:

<%= simple_form_for @person do |f| %>

  <%= f.input :name %>
  <%= f.input :email %>

    <%= simple_fields_for :diseases do |my_disease| %>
      <%= my_disease.input :description %>
    <% end %>

  <%= f.button :submit %>
<% end %>

Thanks a lot for your help.