On a Friday afternoon last month, Niti Mehta, a second-grade teacher at Shady Grove Elementary, began to “shake like a leaf.”

A week earlier, she had the sniffles and nasal congestion, but no fever or shivering.

Trembling, she went home, turned up the heat and got under the blankets. Over the weekend, her condition worsened with nausea, overwhelming weakness and body aches.

Is it the flu, a cold or a bacterial illness?

Often, patients and doctors are unclear whether a respiratory illness is caused by a common cold, the seasonal flu or a bacterial infection. Experts use certain clues to distinguish among the three.

The common cold causes a runny nose, nasal congestion and low-grade fever. The flu is a much-worse version of a cold with high fever, body aches, extreme tiredness and a dry cough. A bacterial respiratory illness often occurs in individuals with underlying lung disease like emphysema or in those who had a recent viral illness. The symptoms of this kind of infection include cough and sputum production.

It’s important to distinguish among the three causes because the treatment for each is different. For a cold, rest and fluids are sufficient. Medicines like Tamiflu and Relenza may be helpful for the flu. For a bacterial respiratory illness, antibiotics like amoxicillin and ciprofloxacin generally lead to a quick recovery.

Often it is a good idea to call your doctor’s office and describe the symptoms. Sometimes, the doctor will ask you to come in, and at other times might just prescribe

medicine over the phone.

Mehta was diagnosed with the flu by her doctor over the phone, and she was started on Tamiflu within 24 hours of her symptoms. Antiviral influenza drugs can make flu symptoms go away a bit sooner. But they help only if they are taken within 48 hours after a person becomes ill. They can cause nausea and other adverse effects, too. Otherwise, rest, fluids and symptomatic treatment are suggested.

It’s flu season

According to the Centers for Disease Control and Prevention, flu season begins in November and ends in March, but the peak for illness occurs in January and February. The data show that eight of every 100 visits to select clinics during these months are for influenza-like illness.

The CDC flu-monitoring map, which resembles a weather map, shows that during the first week of January, flu activity in Tennessee was a “green” — still minimal.

Dr. Helen Morrow, health officer and pediatrician for the Shelby County Health Department, expected the number of flu cases to increase over the coming month.

In a given season, 5 percent to 20 percent of the population will get the flu, and in the past, the number of deaths due to flu-related complications have been as low as 3,000 and as high as 49,000.

“It’s hard to predict what the season will be like this year.” Morrow said. “People don’t realize that the single best way to protect from the flu is the flu vaccine.”

How effective is vaccine?

Morrow cautions the vaccine is not 100 percent effective. Often, the effectiveness of the vaccine depends on the age and health of the patient, as well as the similarity or “match” between the influenza viruses in the vaccine and those spreading in the community. Patients who are over age 65 tend to mount a weaker immune response to the standard flu vaccine, so now the Fluzone High-Dose vaccine is available for older people.

As for the overall effectiveness of the vaccine, a randomized study found that during the 2007-2008 flu season the inactivated influenza vaccine (the shot) protected seven out of 10 people who got the vaccine from influenza.

The most common reason people do not get the flu vaccine is a misconception that they will get the flu from the vaccine. Others do not trust the vaccine.

Studies show that among healthy working adults, the vaccine decreased upper-respiratory illness by 25 percent and absenteeism by 36 percent due to all illnesses.

Yet experts warn that not just absenteeism but also “presenteeism” is a problem with the flu. Presenteeism means people show up at work or school even when they are ill. A 2006 survey of human resources managers found that 56 percent felt that presenteeism had become a problem.

Often, people don’t know when they should stay home. If you have a fever above 100.5 degrees or a cough with significant mucus or phlegm, severe sore throat or any form of rash, vomiting or diarrhea, stay home. If you have sniffles, post-nasal drip, mild sore throat or a dry cough, you likely are not contagious and you can safely go to work or school.

Studies show that one person with the flu can spread it to others who are up to 6 feet away. At home or at work, you can avoid spreading infection by not shaking hands, washing your hands frequently or using hand sanitizers, and avoiding touching your mouth or face. Also you should cover your mouth and nose when sneezing and cough into the crook of your arm, not in your hands.

How long are you contagious?

Mehta had gotten ill on a Friday, so she rested over the weekend. Yet, she still could not muster up enough energy to go to work until a few days later. That worked out well, since the flu virus is contagious from one day before the symptoms to five to seven days after.

But there was a way to avoid getting ill in the first place.

“Even with free vaccination at the health department, few have taken them. People don’t appreciate the disease until they get it,” Morrow said.

Mehta, who was not vaccinated, said, “After this illness, I am definitely getting vaccinated every year.”