If an employee suffers an illness and the doctor writes a medical certification that the employee is sick, who pays for this sick leave?

Generally, federal labor laws do not require employers to provide sick leave or pay sick time off. However, if employers do provide sick leave, the Family and Medical Leave Act (FMLA) permits an eligible employee to substitute paid leave for the unpaid leave to the extent the employer's usual requirements for the use of sick/medical leave are met and the sick leave qualifies as FMLA leave. Employers are also permitted to designate paid leave as FMLA leave assuming all the conditions are met.