Anyone who has had a bad case of the flu knows the feeling: Your brain feels fogged over and thinking is all but impossible.

People can also experience irritability, a desire to be isolated from family and friends, insomnia, daytime sleepiness, fatigue, and in some cases hallucinations.

Researchers have shown that this “sickness behavior” is caused by activation of the brain’s immune system, releasing high levels of pro-inflammatory cytokines and glutamate.

When the illness subsides, cytokines and glutamate levels also subside and symptoms clear up.

In older people with infections such as the flu, inflammation is more intense and takes considerably longer to subside.

This is because the microglia in their brains are in what is referred to as a “primed” state, which makes them react more intensely when the body’s immune system is activated.

When older people develop infections and suffer from hallucinations, the risk of developing dementia in the near future goes up tremendously.

Again, this is because they have primed microglia and an overly intense inflammatory brain reaction that persists much longer than it does in a younger person’s brain.

Vaccinations imitate infections but differ in a few important ways. Most infections enter by way of the gut or respiratory mucous linings.

Vaccines, on the other hand, are injected into muscle or into the deeper layers of the skin — which is a very abnormal route of entry into the body.

Vaccines cause the immune system to react for a much longer time than do natural infections; they also may interfere with the shutdown process of immunity, which is vital to health.

Finally, vaccines are known to cause activation of the brain’s microglia cells, leading to brain inflammation and excitotoxicity — sometimes for very long periods.