You pay income tax on your taxable income. If you have less taxable income (because you are out of work or on reduced pay sick leave) you pay less tax. There is no special tax consideration for the fact that you were out of work for medical reasons.
Medical expenses are allowable as an itemized deduction, and may decrease your tax depending on the amount of your medical expenses and other itemized deductions (like donations to charity, state income taxes, and mortgage interest). You can include your out of pocket costs as deductible expenses when you pay the provider** -- don't include costs that were reimbursed by insurance or a tax-free medical savings account.
**You deduct medical expenses when you pay the provider. If you pay $5000 to the hospital using a credit card and pay the credit card off $100 per month, you can deduct the $5000 payment when you make it. If you make a deal to pay the hospital $100 per month, you can only deduct the payments you actually made during each tax year.
