Communicable diseases are caused by pathogens passed between humans through various means of transmission. The common cold is the most common communicable disease, caused by one of over 200 viruses transmitted through direct contact or contaminated surfaces. Symptoms include congestion, cough, headache, and fatigue. While no cure exists, rest, hydration, and nutrition can help the body fight the virus. Influenza is also highly contagious and spreads similarly, requiring medical consultation if symptoms are severe. Pneumonia is a serious lung infection that may develop from untreated colds or influenza.