What are Infectious Diseases?
Understanding the invisible world of pathogens
Viruses, Bacteria, and Other Microscopic Threats
Infectious diseases are caused by microscopic organisms – known as pathogens – that invade
a host and disrupt its normal biological functions. These pathogens include viruses, bacteria,
fungi, and parasites (such as protozoa and helminths). Viruses are ultra-small agents made
up of genetic material encased in a protein shell, and often enveloped in a lipid membrane.
Unlike bacteria, they cannot reproduce on their own: they must hijack a host cell’s machinery to replicate. Bacteria, in contrast, are self-sufficient single-celled organisms. Although many bacteria are beneficial or harmless, some species are pathogenic, causing illness through toxins or tissue damage. Other infectious agents include fungal organisms (e.g., Candida) and parasitic protozoa like Plasmodium, which causes malaria. There are also prions, infectious misfolded proteins with no genetic material, responsible for rare but fatal neurodegenerative
+ disorders.
How Infections Spread
Pathogens spread through various mechanisms depending on their biology and context.
Respiratory viruses (e.g., influenza, SARS-CoV-2) are airborne and transmitted via droplets.
Others are spread by direct contact with bodily fluids (HIV, Ebola), via contaminated food or
water (hepatitis A, enteric bacteria), or by vectors like mosquitoes or ticks (dengue, malaria, Lyme disease). Contaminated surfaces can also harbor pathogens, facilitating indirect transmission when people touch their face or wounds without proper hygiene. Epidemics begin when a pathogen is transmitted from person to person in a chain of infection, leading to a local outbreak. If unchecked, this can escalate into an epidemic, and when it crosses borders and affects multiple continents, a pandemic.
Why Do New Epidemics Emerge?
Many emerging pathogens originate from zoonotic spillover – when a virus or bacterium jumps from animals to humans. This has occurred with SARS (2002), Ebola (2014), and Zika (2015–2016). Environmental changes such as deforestation, urbanization, and climate change increase human exposure to animal reservoirs. Global travel and trade further accelerate the risk of rapid dissemination. While most zoonotic transmissions are contained, some adapt to efficient human-to-human transmission, triggering widespread outbreaks with severe health, social, and economic consequences
Epidemic vs. Pandemic
An epidemic is a sudden increase in cases of a disease within a specific region. A pandemic is an epidemic that spreads globally. Importantly, the term “pandemic” does not refer to the severity of the disease, but to its geographic reach. For example, a pandemic flu strain may be mild, while an epidemic of Ebola, though more lethal, may remain geographically contained. The COVID-19 pandemic, declared in March 2020, showed how quickly a local outbreak can become a global crisis.
Preparing for Future Threats
The COVID-19 crisis highlighted the urgent need for global pandemic preparedness. Key strategies include surveillance, rapid diagnostics, genomic sequencing, and global data sharing.
While vaccines are critical, they are not always available early in an outbreak. Thus, broad-spectrum antiviral drugs – especially those that can be rapidly deployed and manufactured – are essential tools. Projects like AVITHRAPID, funded by the European Union, aim to develop a portfolio of antiviral agents targeting a wide range of viral families. By investing in proactive strategies, we can better contain the next Disease X before it becomes the next pandemic.