Final answer:
Diarrhea is the leading cause of illness in children under five years old, with malnutrition and poor water quality being significant contributors. Viral pathogens like rotavirus are common in the developing world, and the availability of healthcare greatly influences outcomes.
Step-by-step explanation:
The leading cause of illness in children under five years old is diarrhea. Diarrheal diseases, often due to inadequate water quality, sanitation, and hygiene, remain a significant cause of morbidity and mortality in this age group. Factors such as malnutrition also play a crucial role, as it not only contributes to mortality but is tightly linked with the incidence of diarrheal diseases. There are a variety of pathogens that can cause diarrhea, with viruses like rotavirus being particularly common in the developing world.
Although respiratory conditions like asthma can lead to significant healthcare use in children, they do not surpass diarrhea as a cause of illness worldwide for children under five. Moreover, the availability of healthcare varies substantially between high-income and low-income countries, which affects the prevention and treatment of these conditions.