According to [1], people are contagious for about one week, starting from one day before they feel any symptoms. It is actually more productive to send the sick people home (or let them work at least part-time from home) to prevent others from getting sick and also having to take time off.
I've been fortunate enough to work in environments where this was acknowledged and practiced, but I'm always dismayed when hearing from friends/relatives who are required to put in their "time of presence" and get half of their coworkers sick.
[Edit] Last point: for the common cold (virus), there's no need for a doctor visit, yet you still need rest and symptom control with OTC meds.
[1] http://www.webmd.com/cold-and-flu/how-long-flu-contagious
I'm just saying that when there is an extended absence from work, it's reasonable that your employer request a note. Now maybe 6 or 7 work days doesn't meet the definition of extended for some people, but for me it does.
And as stated elsewhere, this is not about a nosy manager getting their employee's health information. This is about a manager exercising their right to know that the employee is not at work for a legitimate reason. I've had these types of notes before, and they typically just have a date range of when you should not be in the office.