A colleague returns to work while still suffering from the flu.
The spiked surface of the influenza virus (dark blue) fits like a chemical key to unlock a respiratory cell and hijack its reproductive mechanisms. Inside the virus are the genetic strands (red) that it will use to reproduce inside the infected cell.
You notice you're feeling achy, your throat is scratchy and you feel feverish.
This sequence of electronmicrographs shows a flue virus infecting a cell. In the top frames, the rounded virus attaches to the respiratory cell (blue), which then folds around it, bringing it inside. In the lower frames, the virus has fully penetrated inside the cell, where it will reproduce itself. In six hours the cell will have reproduced 1,000 new viruses. Twenty-four hours after the virus infects the first cell, a trillion new viruses may be present in the body.
The flu is in full swing - fever, body aches, exhaustion, sore throat and headache.
When the new viruses are completed, they "bud" from the surface of the infected cell to begin the process again. Within 48 hours of the first viruses entering the body, viruses are overwhelming respiratory cells. Your symptoms are severe - in part because the body's immune response has begun in earnest.
Your symptoms continue - fever, chills, body aches, exhaustion, and pounding headaches, and a dry, persistent cough.
When the body recognizes the virus, a complicated response begins. A T-lymphocyte, a white blood cell (lower right) recognizes an antigen (yellow), with a receptor on the T-cell's surface (green). It becomes "activated," increasing in size and synthesizing Interleukin-2 (blue and purple ovals), a chemical that induces fever, stimulates inflammation in the region, and attracts a host of other immune cells. The T-lymphocyte also produces antibodies (red, y-shaped molecule), chemicals that bind to the virus and make it powerless.
Your symptoms are abating. Fever is down and body aches and headache have subsided. You're still tired and coughing, which may continue for another week.
The tide has turned and the immune system has gained the upper hand. The T-lymphocytes attract other immune cells to join the fight, including macrophages (or "big eaters"). Macrophages engulf and destroy invading viruses and bacteria, dead cells, and a host of debris, as this macrophage engulfs a yeast cell. They also summon killer T-cells, which release proteins that literally destroy invaders, like something out of science fiction.