Bacteria are living things that have only one cell. Under a microscope, they look like balls, rods or spirals. They are so small that a line of 1,000 could fit across a pencil eraser. Only a few kinds of bacteria cause disease; these are called pathogens.
The most common fatal bacterial diseases are respiratory infections. Bacteria can cause disease by producing harmful substances called toxins, invading tissues, or doing both. Bacterial infections may be treated with antibiotics, though some bacteria are resistant or immune to antibiotic treatment. The more often antibiotics are used, the more likely resistant bacteria are to develop. Vaccines can provide resistances to bacteria.