The first day you think: "Dang, my head hurts. And my stomach isn't feelin' too good either. But that report is due tomorrow, I'd better get to work."

The second day you think: "I wish I hadn't gone to work yesterday. I coulda used the rest. Now all I can do is lie here in bed trying not to hurl."

The third day you think: "I wonder whether I should go see a doctor?"

Maybe yes. Maybe no.

On its Web site, health experts at Grand Itasca Clinic and Hospital in Minnesota say the following questions are useful in determining whether to see a doctor:

Is your pain unresponsive to over-the-counter medications?

Have you had a fever above 102 for two or three days (adults)?

Is your illness getting progressively worse?

If you answered yes to these questions, you should make an appointment with your physician.

On the health information site WebMD.com, Sharon Horesh, instructor of clinical medicine at Emory University School of Medicine in Atlanta, elaborates on these rules of thumb.

She says sniffles without aches and fever are probably just allergies and not cause for a visit to the doctor.

However, if you're soaking your clothes with sweat, you probably have a fever. Take your temperature. If the fever doesn't abate in two or three days you may have the flu or an infection that requires a doctor's care.

Horesh says that if, in addition to a fever, you see white patches on your tonsils, you may have strep throat and may need an antibiotic. You should see a doctor.

If you have a cough without other symptoms, such as aches or fever, you are most likely suffering allergies. If you're experiencing a deep cough that produces green mucus and makes you short of breath, Horesh says your condition requires a doctor's diagnosis. You may have bronchitis or pneumonia.

Severe earaches that diminish your hearing and don't go away in a day or two probably mean you have an ear infection, which will require an antibiotic. Horesh says ear infections don't typically go away on their own.

If your face hurts, including pain around your eyes and cheekbones, you may have a common cold but it could be a sinus infection, in which case you should see a doctor. You may need an antibiotic.

Vomiting and diarrhea are symptoms of a variety of maladies. If they occur six to 12 hours after eating, you may have food poisoning. After throwing up, you should feel better. If not, Horesh says, you may have a stomach virus. And because viruses cannot be treated with antibiotics, you'll just have to tough it out for a day or two. Drink plenty of water and get plenty of rest. Avoid solid foods. If your condition doesn't improve, see a doctor.

The folks at Grand Itasca Clinic and Hospital clearly have a great deal of experience in these matters, because they have a prepared answer to the frequently asked question: "Why can't I call the doctor's office and tell the nurse or doctor what's wrong and get a prescription?"

Because doctors need more information to make a diagnosis, that's why. To correctly assess your condition, a doctor may need to conduct a physical examination, order lab work or X-rays or just get a good look at you.