Final answer:
Diseases are caused by microorganisms such as bacteria and viruses according to the germ theory of disease, which was clearly stated by Girolamo Fracastoro in the 1500s and later supported by the microscopic observations of Anton van Leeuwenhoek.
Step-by-step explanation:
The Germ Theory of Disease
According to the germ theory of disease, disease is not caused by decaying matter, chemicals, miasma, or proximity to decomposing matter. Instead, diseases are caused primarily by microorganisms. These can be bacteria, viruses, or other types of microbes. The theory was clearly stated in the mid-1500s by an Italian physician named Girolamo Fracastoro, but the idea was disregarded until evidence accumulated, including the direct observation of microorganisms by Anton van Leeuwenhoek in the 17th century.
Pathogens, which are disease-causing microorganisms, grow, reproduce, and make their hosts sick. They can spread contagious diseases from person to person. Despite the challenges of drug-resistant pathogens and factors like overpopulation and inadequate medical care, which contribute to the spread of infectious diseases, the germ theory stands strong, completely transforming our understanding of how diseases are spread and how they can be controlled.