Freezing temperatures do not eliminate viruses; instead, they often preserve them, allowing them to remain infectious for extended periods. Viruses are microscopic infectious agents that can only replicate inside the living cells of other organisms. Understanding how viruses react to cold is important for public health and safety.
How Freezing Affects Viruses
When viruses are subjected to freezing temperatures, their activity and metabolic processes slow down or stop. Deep freezing is a common method used in microbiology laboratories to preserve virus samples for long-term storage and study. While ice crystal formation during freezing can damage delicate viral structures, this damage is often insufficient to render viruses non-infectious upon thawing.
Why Freezing Doesn’t Kill Viruses
Viruses are fundamentally different from living organisms like bacteria, explaining their resistance to freezing. Unlike bacteria, viruses lack cellular machinery and do not carry out metabolic processes independently. They consist primarily of genetic material, either DNA or RNA, encased within a protective protein shell called a capsid, and sometimes an outer lipid envelope. This simple structure makes them resilient to extreme conditions, as they lack complex, water-filled cellular components that would be ruptured by ice crystal expansion, which typically kills bacteria. Freezing cannot “kill” viruses, but rather maintains their infectious potential.
How Viruses Are Inactivated
To inactivate viruses, methods that disrupt their structural integrity or genetic material are necessary. Heat is a widely used method; for instance, temperatures around 60°C for 30 minutes can inactivate many viruses by denaturing their proteins and damaging their nucleic acids. Boiling food, for example, is an effective way to destroy viruses.
Chemical disinfectants, such as alcohol-based sanitizers, bleach, and hydrogen peroxide, work by altering or breaking down viral proteins and dissolving their lipid envelopes. These chemicals disrupt the virus’s ability to attach to and infect host cells. Ultraviolet (UV) radiation, particularly UVC light, also inactivates viruses by damaging their genetic material, preventing replication. This method is used in sterilization applications, including in hospitals for surface disinfection.
Practical Implications for Virus Survival
Understanding virus survival in cold environments has several practical implications. Viruses can survive on frozen foods for extended periods, as seen with outbreaks linked to contaminated frozen produce or meat. This highlights the importance of proper food handling and cooking.
Viruses can also persist on cold surfaces like doorknobs or in refrigerators. Laboratories routinely store viruses at ultra-low temperatures, such as -80°C or in liquid nitrogen, to maintain their viability for decades. This demonstrates that freezing is a preservation technique, not a method of elimination. To prevent viral transmission in daily life, relying on freezing is ineffective; instead, consistent use of proper disinfection methods, thorough cleaning of surfaces, and adequate cooking of food are necessary.