The world's second nuclear explosion was achieved with uranium-235. This isotope is unstable and fissions when struck by a neutron. It is, however, found in natural uranium at a concentration of only 0.7 percent. To be useful in nuclear weapons, the concentration must be increased.
Introduction. Influenza, or flu, is a caused by a virus infecting the respiratory system, meaning your nose, throat, bronchial tubes, and lungs. Flu symptoms are usually more severe than those of the common cold and are more likely to affect other parts of your body.
Flu (Influenza). The flu is a common infectious disease caused by influenza (flu) viruses. The flu usually affects a person's breathing system. Seasonal flu generally occurs in the fall and winter and is caused by human influenza viruses.1  Flu Frequently Asked Questions.[ 2  English ] [ Spanish ] [ Chinese ].he usual signs and symptoms of the flu are cough, sore throat, runny or stuffy nose, body aches, headache, chills, and feeling very tired. Most people also have a fever. Others may throw-up and have diarrhea.