Figure 3.7 According to the endosymbiotic theory, mitochondria and chloroplasts are each derived from the uptake
of bacteria. These bacteria established a symbiotic relationship with their host cell that eventually led to the bacteria
evolving into mitochondria and chloroplasts.
- What does the modern endosymbiotic theory state?
- What evidence supports the endosymbiotic theory?
The Germ Theory of Disease
Prior to the discovery of microbes during the 17th century, other theories circulated about the origins of disease. For
example, the ancient Greeks proposed the miasma theory, which held that disease originated from particles emanating
from decomposing matter, such as that in sewage or cesspits. Such particles infected humans in close proximity to
the rotting material. Diseases including the Black Death, which ravaged Europe’s population during the Middle Ages,
were thought to have originated in this way.
In 1546, Italian physician Girolamo Fracastoro proposed, in his essayDe Contagione et Contagiosis Morbis, that
seed-like spores may be transferred between individuals through direct contact, exposure to contaminated clothing,
or through the air. We now recognize Fracastoro as an early proponent of thegerm theory of disease, which states
that diseases may result from microbial infection. However, in the 16th century, Fracastoro’s ideas were not widely
accepted and would be largely forgotten until the 19th century.
In 1847, Hungarian obstetrician Ignaz Semmelweis observed that mothers who gave birth in hospital wards staffed by
physicians and medical students were more likely to suffer and die from puerperal fever after childbirth (10%–20%
mortality rate) than were mothers in wards staffed by midwives (1% mortality rate). Semmelweis observed medical
students performing autopsies and then subsequently carrying out vaginal examinations on living patients without
washing their hands in between. He suspected that the students carried disease from the autopsies to the patients they
88 Chapter 3 | The Cell
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