At that time, the German physician, Robert Koch , established a set of rules which could be used to test if a disease could be attributed to a microorganism. These rules, known as Koch's postulates, are still important among modern procedures used to establish the relationship between diseases and the microorganisms which cause them. Application of Koch's principles have been important in establishing the relationship between viruses, bacteria, protozoans and the diseases they cause. Studies have demonstrated that viruses and microorganisms which cause them are spread through the air, food, water, insect vectors, and sexual contact. All of these species of bacteria may be controlled or cured by antibiotics but development of antibiotic resistance is proving a serious problem.
Viruses are in all species of organisms.
The diseases were known about for a long time before the viruses which cause them were identified. As noted earlier, viruses have caused innumerable losses to humanity over time. Cholera, tetanus, whooping cough, diptheria, leprosy, bacterial pneumonia, etc. are important diseases caused by bacterial pathogens. Members of the Kingdom Protista are also causative agents of numerous diseases.