How was the flu so deadly in Season 4 of The Walking Dead?

From Wikipedia

Influenza spreads around the world in seasonal epidemics, resulting in about three to five million yearly cases of severe illness and about 250,000 to 500,000 yearly deaths,[18] rising to millions in some pandemic years. In the 20th century three influenza pandemics occurred, each caused by the appearance of a new strain of the virus in humans, and killed tens of millions of people.

So by itself, Influenza is pretty nasty. Add to that the fact that everyone is already quite unhealthy and most likely hurt in one way or another... massively compromised immune systems... and the flu would be even worse. No medication? That's adding insult to injury. Now... the detail we don't know for sure... since everyone is infected, does that compromise their immune system even more?

I don't think the virus was just supposed to be a regular flu, I think it was supposed to be some form of zombie Swine Flu, and here's why:

• The virus was zoonotic as it infected and killed Violet, the pig they were keeping, just before the first infections in the prison:

  

• Zombies are also infected with the virus:

  

• In 2009 (just around when season 4 was being written) there were a lot of media panic about Swine Flu.

• Hershel didn't get sick, despite being in contact with everyone that was, which fits with this:

  Existing vaccines against seasonal flu provide no protection against swine flu, A study at the U.S. Centers for Disease Control and Prevention published in May 2009 found that children had no preexisting immunity to the new strain but that adults, particularly those over 60, had some degree of immunity

• As Wiki states:

  Some patients experience severe respiratory symptoms. (The H5N1 virus is also able to infect cells deep in the lungs with receptor type a2-3 but cannot infect cells with receptor type a2-6 making it less contagious than H1N1/09.)

  Which makes sense as in the show people die from coughing up blood.

Severe Leptospirosis - can be life-threatening. There is a risk of organ failure and internal hemorrhaging. This occurs when the bacterium infects the kidneys, liver and other major organs. Experts are not sure why some patients develop the severe form - people who are already very ill, such as those with pneumonia, young children under five, and elderly individuals are more likely to suffer from severe Leptospirosis. it is bacterial ( treatable by antibiotics)it is commonly transmitted when animal urine and wastes end up contaminating drinking water.....(.and where is the pickup line for all their water??? at the bottom of the hill below the pig pens..)..there is a lot of old knowledge that our urban dwellers will have forgot so diseases that killed millions in the 16th and 17th centuries are going to make a comeback

The modern flu is fought through maintaining a stress free environment, bed rest aka inactivity, and keeping yourself clean, aka preventing secondary infections from overwhelming an already occupied/compromised immune system. Secondary infections cause the immune system to divide it's attention. Stress, a mental component, has a high psychosomatic component, resulting in a handicapped immune response. Mind over matter, essentially the placebo effect, is real. Activity wears down body resources, like glucose levels, which are crucial to fighting infections. A tired, afraid person will take longer to fight an infection then someone well rested and calm. It's not just that it's the flu, but the entirety of the circumstances that increases the death toll.

Remember, everyone in the Walking dead is already infected with the Zombie virus. As long as they die with their heads intact they will reanimate. As such, the immune system is already dealing with more than one infection at the time they get the flu. Immuocompromised

First they are already infected with the walker illness. So their immune systems are compromised. It's like how the bite from a walker only kills you, it doesn't cause the walkerism, it's all the bacteria and fungus in their mouths from dying and eating raw meat that will infect a person and kill them: like a kamodo dragon bite.

The flu that is spreading was going around anyway. But with the weakened immune systems they cannot fight it off very well. Like what jfm said about stress and overworking the body. Also Hershel, Maggie and Beth are likely immune because they grew up with farm animals (how they got the flu in the first place) and probably already got the flu a long time ago.

Well Another suggestion is the Actual Infection mentioned in the program which is actually called Pneumococcal by what Dr S Said.

Where it is not actually classed as the flu.

Thus this is true to real life scenario. However this is not the case. Population is infected by the disease that turns you into a walker. So by this statement no matter what your intake is the disease strand mixed with a flu straind your body is weak. Nothing to have to prevent it