When your body is invaded by things like a virus, the immune system sends out white blood cells (also called leukocytes) to find the infection. The white blood cells release a chemical called cytokines, which is what causes the joint pain seen not just in the flu but with many inflammatory/autoimmune conditions like Arthritis, lupus,inflammatory bowel disease, etc.
I have copied this answer from user njdocisin on Yahoo answers who clearly knows what she's talking about.
You have the flu when your body has been infected with an influenza virus.
It is an infectious disease, so you "caught" it from someone else who was infected with it. The most common way it is "caught" is from direct bodily contact with another person, such as shaking hands, holding hands, kissing, etc. Or you can get it if someone is coughing or sneezing near you without covering their nose and mouth, so the respiratory droplets come in contact with you. You can also get it by just touching something an infected person touched.
Prevention methods include flu vaccinations and frequent hand washing. See more in the related questions below.
Your body is working hard Behind the Scenes to fight off the infection. It uses energy even while you rest, so you don't notice the loss, and consequently are more tired than you should be.
Because your body is tired and you lack nutrients
because you denounced Satan
I had the flu 3 weeks ago followed by complications of sinus and ear infections. Became seriously ill and to this day feel weak. Although I feel less tired each day, it still seems that I have a limited amount of energy each day.
Getting the flu does not make a person stronger. A person that has the flu becomes sick and weak. Many people die each year from the flu.
Kari got the flu from having a weak immune system. Kari died because they did not treat the flu in time.
Love flu is not really influenza (flu). It is something you feel when you like someone. They call it a flu because a flu spreads and love happens to everyone.
Yes, it is. When you have the stomach flu, you generally can't keep food or water down. This lack of fuel for the body makes everything weak, from the body to the mind. So, yes it is normal for you to have weak lets when you have the stomach flu. However, if the weakness persists after a few days after the stomach flu clears up and you've been able to hold down food and water, you might want to see a doctor.
no it is only a flu. although some people may die, but that only means they have a weak immune system.
you probably have a cold or a flu, I feel like this when I am ill.
After a week in bed with the flu, his face was ahsen and he felt weak.
What do I feel sick and weak after my menstrual cycle is over
There are a few reasons why one would feel weak beside one's friend. For example, the friend may lift more weight and thus making one feel insecure or weak.
why a person that have not eaten in a week feel weak is because without food you will not have energy to move at all that's why they are weak.
you feel like you hav the flu