HIV infection can occur when an individual is exposed to HIV virus, and the infection usually involves sexual contact or direct blood contact, either through an open wound or penetration of skin by a contaminated needle. Then the virus will enters to the body's lymph nodes, then gets into the cells of the immune system called T cells. Then comes the "cat and mouse war". The virus and immune system will use complex strategy to fight against each other until one party was defeated.

Perhaps the greatest of these tricks are from HIV, which incorporates itself into the DNA of the host T cells so that it is hidden in the protected sites from immune system, such as the central nervous system; this makes the body unable to clear the HIV virus completely, and if the patient is not effectively treated, then the virus can still be detected in the patient's blood. The cells responsible for attacking the virus will produce new virus particles, and attack other infected T cells; if not treated the patients ' immune system will be destroyed, and some rare infections or a syndrome called immunodeficiency syndrome (AIDS) are almost fatal to the patient.

In contrast, the influenza virus spreads through the respiratory droplets. When the infected individuals cough or sneeze, the virus particles will be spread; when the droplets are inhaled by a new host, the virus will bind to target receptors on the respiratory tract surface, and replicate in the cells of the respiratory tract.

Unlike HIV, the influenza virus can not insert itself into the host's gene, so the body's immune system can detect the infection, shut down viral reproduction, while clear the infected cells. The medications can help this process, but they are usually unnecessary, as the body's immune system is usually ble to cure influenza independently. Previous influenza infection or vaccination will provide the body some immune memory, and also provide protection and faster immune clearance of the virus.

When the individual exhibits flu-like symptoms, the influenza virus can be found in the body's blood in a very short time, especially in very serious disease cases. Although bloodstream infections are usually not tested for influenza virus infection, blood donation is often not recommended if blood donors are uncomfortable and have flu-like symptoms.