The smallest pathogens, which are also responsible for illnesses like colds and chicken pox, are viruses. Viruses are a fraction of the size of bacteria and are transmitted from one person to another in a number of ways, including direct contact, sharing of drinks, touching infected surfaces, and even simply breathing. Sicknesses caused by viruses can range widely in severity from a common cold to a life-threatening condition like AIDS.
Viruses are typically controlled through antiviral medications and vaccinations. The former treatment can help to treat an existing viral infection, while the latter is intended to prevent the infection from occurring in the first place. These tiny pathogens are responsible for hundreds of diseases around the globe, and are often highly contagious.