If the note shows that the employee cheated you get a warning. If the action is repeated then a doctor's note can be required from day one.
Start with good faith, but add some flexibility.
Personally, although I've fortunately had much need to take an appreciable number of sick days, I favor them being separate because as others note, especially with a relatively limited pool of PTO days, there's a lot of incentive to work sick.
Well, that makes sense. When I'm stressed at work, I often get sick at the weekend. Makes sense it could start early, or leak into the next week.
If you have a joined pool of 60 PTO days per year, that means more days off than separate pools of 30 vacation days + up to 30 sick days. All else being equal, of course an employee would prefer to have more days off. However, all else would not be equal. The PTO days are a cost to the employer, which means the employer can afford to shell out less in other forms of compensation. Think of the job market as a market where employers compete for employees (and vice versa, but that's tangential to the point here). Employers offer different forms of compensation to employees: salary, bonuses, paid vacation days, etc. If the government suddenly mandated that employers must give all employees more vacation days, then the employers could afford to pay out less in other forms of compensation. Sure, in the short run the employer would pay out more to employees and be less profitable, but over time they would balance out this cost by paying less in other benefits.
In other words, if you take the perspective of an employee with a 5+ year time horizon, the total amount of compensation received from the employer would not be expected to increase as a result of those government-mandated extra vacation days. What would change, however, is the incentive of people to go to work sick and infect others. If sick days cost you money, then you are incentivized to go to work sick. As a result, more people get sick. That would be bad. So from a risk/reward pespective, we have a reward of ~0, and a risk of more sickness.
Everyone getting the same combined amount of day mitigates the above, with the understanding that might be 45 PTO days instead of 30 vacation + 30 sick. This would seem to leave the problem of people who are genuinely sick for longer 15 days, under the 30+30 they'd not be dipping into their vacation days as quickly.
There will always be some number of people who run out of sick days, no matter how many you have. Maybe the policies around short-term disability should be adjusted, and the more drastic situations handled with that (requiring doctor visits, etc).
I'm currently reading "Bullshit Jobs" by David Graeber, something that rings a lot of bells for me. I'll admit to having an very questioning attitude about workplace culture lately :-)
Another thing that is important to point out is that leave days are sometimes an earned entitlement ultimately paid out to you if not taken, whereas sick leave isn't.
It's kind of insane to me that they can just be taken away like that.
[1] http://ask.legalsolutions.thomsonreuters.info/cc-nto-vacatio....
However, historically, when the two were commonly separate, the sick time would often actually be a very significant chunk (maybe a month) with the understanding that actually burning through most or all of your sick time would be a rare event because of an operation or whatever.
Just to throw out rough numbers, you might have 4 weeks of a combined PTO pool today. 20 years ago you might have had 3 weeks of vacation time and 4 weeks of sick time.