Disease-causing microbes (bacteria, viruses, and parasites) invade your body through your mouth, eyes, genital openings and wounds. Once inside, these germs damage tissues while using energy from your cells to reproduce and spread. Infectious diseases spread from person to person through the air in sneezes or coughs; through bodily fluids like sweat, saliva, semen, vaginal fluid or blood; through food and water; or by insects and ticks.
The germs that cause infectious diseases have produced suffering throughout history. Over the last 200 years, vaccines and antibiotics have made great strides in combating these scourges—so much so that in December 1967, Surgeon General William H. Stewart declared victory over infectious diseases. Unfortunately, no one told the germs. New strains emerged, often with resistance to existing treatments. Today, infectious diseases remain the leading cause of death around the world. Even in the US they are the third-biggest killers, behind cancer and heart disease.