Patients often ask me: Can I get the flu twice in one season? The answer, unfortunately, is that you can, though rarely. It’s much more common to mistakenly think you have the flu when in fact you’re suffering from another illness.

There are four main strains of the flu circulating this year, as in most years. If you catch one strain, you’ll develop an immunity to that, but can still get one of the other strains.

As the U.S. Centers for Disease Control explains: “There are four types of influenza viruses: A, B, C and D. Human influenza A and B viruses cause seasonal epidemics of disease almost every winter in the United States. The emergence of a new and very different influenza A virus to infect people can cause an influenza pandemic. Influenza type C infections generally cause a mild respiratory illness and are not thought to cause epidemics. Influenza D viruses primarily affect cattle and are not known to infect or cause illness in people.”

The influenza A virus originated with birds. A strain of influenza A called H3N2 has predominated so far this season and causes severe illness. But the H1N1 strain of influenza A and the two strains of influenza B are now starting to circulate more commonly.

Here’s one way you can tell if you likely have the flu: If you’re walking around one minute and have to lie down the next because of the rapid onset of fatigue or body aches and fever, that’s likely the flu, especially right now.

This year’s flu shot does a better job of matching up with the latter three strains, so it is still a very good idea to get vaccinated if you haven’t gotten a flu shot already. More on that below.

Surprisingly, you may think you’re coming down with the flu for the second time this season but be mistaken. That’s become some illnesses mimic some flu symptoms. It’s hard for the average person to tell the difference.

As a result, millions of Americans who think they had the flu this season really didn’t have it at all. How can you tell if you’re one of them?

For one thing, though the flu is a respiratory virus with symptoms that can vary from mild to severe, the trademark of the illness is a battle with the immune system causing inflammation.

This inflammation is characterized by rapidity of onset, fatigue, muscle aches and fever. Look for these symptoms before you announce that you have the flu. Yes, flu can also give you a sore throat, headache, cough, and a runny nose, but you can also have these symptoms with a cold, sinus infection or strep throat.

In fact, there are a lot of upper respiratory infections being passed around throughout the U.S. right now, some viral and some bacterial. Many concerned patients coming to see me in my office with a bad cough who are convinced they have the flu are actually suffering from bronchitis.

Of course, you can have both the flu and bronchitis at the same time, and your physician needs to be alert to this possibility. Even if your doctor performs a rapid flu test and it’s positive, you can also have a bacterial bronchitis, pneumonia, sinusitis or strep throat. Your doctor has to treat whatever combination has hit you.

The truth is, I don’t always perform a flu test on my patients, because it is only accurate 60 to 70 percent of the time. Luckily, I can often diagnose the flu just by talking to and examining a patient – and your doctor can likely do this as well.

Here’s one way you can tell if you likely have the flu: If you’re walking around one minute and have to lie down the next because of the rapid onset of fatigue or body aches and fever, that’s likely the flu, especially right now.

Patients commonly ask me if they should get a flu shot. I hope you’ve gotten one already, but if you haven’t, my advice is that you should do so now – it’s not too late.

We won’t know how effective the shot is until the flu season is complete, but early indications are that it will keep you from getting the flu about 30 percent of the time. We all wish the protection rate was higher – but 30 percent is better than having no protection at all.

There’s an additional benefit to having the flu shot. It may help decrease severity of the flu if you get sick and make it harder for you to spread the disease to others who may be a higher risk for complications – pregnant women, infants, the elderly and those with chronic diseases.

Believe it or not, most of our flu vaccine batch is still made by inoculating chicken eggs, a process that was first pioneered in the 1950s. In order to grow in an egg, which has an environment suited to bird flus, the flu virus often mutates.

Dr. Anthony Fauci, director of the National Institute of Allergies and Infectious Disease, recently told me that the flu virus unfortunately mutated this season in a way where the H3N2 antigens on its surface are less susceptible to the antibodies produced by the vaccine.

Hemagglutinin is the protein that attaches the flu virus to the surface of your cell, and Neuraminidase cleaves the flu virus (after it has reproduced inside) from your cell so it can jump to a neighboring cell.

Neuraminidase inhibitors (including Tamiflu) are effective anti-viral drugs that decrease the spread, severity and length of the illness once you have the flu. These drugs can also be used to protect high-risk patients from getting the flu in the first place.

Because of the hens’ eggs absurdity, the need for new flu shots every year and the relative ineffectiveness of the yearly shot, it has long been obvious to scientists that we desperately need a universal flu vaccine.

There are several candidates now entering clinical trials in humans. Dr. Fauci said he is excited by a new technique that targets the “stalk” of the hemagglutinin molecule, common to flu of all kinds.

It remains to be seen whether this process generates sufficient immunity to be useful in clinical practice, but it is certainly promising. Whichever universal influenza vaccine candidate is ultimately successful, one thing’s for sure: it will be produced using the cells of mammals rather than chicken eggs and will use modern recombinant genetic technology.

When the mass-produced universal flu vaccine arrives, it will be a great day for public health. The yearly flu epidemic that hits millions of us each year and that millions fear will be squashed and recede into history. That’s something we can all look forward to.