21.2 Virus Infections and Hosts

Viruses can't reproduce on their own. They depend entirely on host cells to copy their genomes and build new viral particles. Understanding how viruses infect hosts, replicate, and spread is central to explaining viral diseases and figuring out how to prevent them.

Virus Replication and Cycles

Steps of virus replication

Every virus follows the same general replication strategy, though the details vary by virus type. The six core steps are:

1. Attachment: The virus binds to specific receptors on the host cell surface (such as glycoproteins or sialic acid residues). This step determines which cells a virus can infect. If the receptor isn't there, the virus can't get in.
2. Penetration: The virus or its genome enters the host cell. This can happen through receptor-mediated endocytosis (the cell engulfs the virus) or through direct membrane fusion. Influenza enters by endocytosis, while HIV fuses directly with the plasma membrane.
3. Uncoating: Host enzymes break down the viral capsid, releasing the viral genome into the cytoplasm. This frees the nucleic acid so it can be read by the cell's machinery.
4. Replication: The host cell's ribosomes, tRNA, and enzymes are used to copy the viral genome and produce viral proteins. Retroviruses like HIV carry their own enzyme, reverse transcriptase, to convert their RNA genome into DNA first.
5. Assembly: Newly made viral proteins and copied genomes come together to form new virus particles. Some viruses self-assemble; others require scaffolding proteins to guide the process.
6. Release: New viruses exit the host cell either by lysis (the cell bursts open and dies) or by budding (viruses push through the plasma membrane, picking up an envelope as they go). Influenza, for example, buds from the host cell membrane. The total number of viruses released is called the viral load, and higher viral loads generally correlate with more severe infections.

Lytic vs. lysogenic cycles

These two cycles describe different strategies a virus can use after entering a host cell.

• Lytic cycle: The virus immediately hijacks the cell and begins replicating. The host cell fills with new viral particles until it lyses (bursts), releasing them to infect neighboring cells. T4 bacteriophage and influenza virus follow this pattern. The lytic cycle is fast and destructive.
• Lysogenic cycle: Instead of replicating right away, the viral genome integrates into the host cell's DNA, becoming a provirus (or prophage in bacteria). The provirus is copied every time the host cell divides, silently passing to daughter cells. Lambda phage and herpes simplex virus can enter this cycle.
  • The provirus can remain dormant for long periods. This dormancy is called latency. Herpes viruses, for instance, persist in nerve cells for years between outbreaks.
  • Environmental triggers like UV radiation, stress, or hormonal changes can reactivate the provirus, switching it into the lytic cycle.

Viral transmission in organisms

How a virus spreads depends on the type of host it infects.

Plant viruses can't penetrate intact plant cell walls on their own. They rely on:

• Vectors such as aphids, nematodes, or fungi that carry the virus from plant to plant. Aphids are especially common vectors.
• Mechanical transmission through contaminated tools, hands, or plant sap. Tobacco mosaic virus spreads easily this way.
• Seed transmission, where an infected parent plant passes the virus to offspring through seeds (e.g., bean common mosaic virus).

Animal viruses use a wider variety of transmission routes:

• Respiratory droplets or aerosols (influenza, SARS-CoV-2)
• Fecal-oral route, often through contaminated water or food (rotavirus, norovirus)
• Direct contact with infected individuals or contaminated surfaces (rabies through bites, herpes simplex through skin contact)
• Vector-borne transmission by insects or other animals (mosquitoes transmit dengue virus; birds carry West Nile virus)

Zoonosis refers to the transmission of viruses from animals to humans. Zoonotic spillover events are responsible for many emerging diseases, including COVID-19, Ebola, and HIV.

Major viral diseases

Plant viral diseases:

• Tobacco mosaic virus (TMV) causes characteristic mosaic-patterned discoloration and stunted growth in tobacco, tomatoes, and peppers. It was the first virus ever discovered.
• Cucumber mosaic virus (CMV) has one of the broadest host ranges of any plant virus, infecting squash, melons, and many other crops. It causes mosaic patterns, leaf curling, and malformed fruit.
• Potato virus Y (PVY) reduces tuber quality and yield in potatoes, making it a major concern for potato agriculture worldwide.

Animal viral diseases:

• Influenza causes respiratory illness in humans and animals. Seasonal flu can lead to severe complications like pneumonia, and pandemic strains can cause widespread mortality.
• Rabies infects the central nervous system of mammals, causing encephalitis (brain inflammation). It is nearly always fatal once symptoms appear, but post-exposure vaccination can prevent disease.
• Feline leukemia virus (FeLV) suppresses the immune system and causes cancer in cats.
• African swine fever virus (ASFV) is highly contagious and often lethal in domestic and wild pigs, with no available vaccine.

Viral adaptations and host interactions

Viruses have evolved several strategies that help them infect hosts and evade immune defenses.

• Viral tropism refers to which specific cell types or tissues a virus can infect. HIV, for example, targets helper T cells because they express the CD4 receptor. Tropism determines where in the body a virus causes disease.
• Antigenic drift involves small, gradual mutations in viral surface proteins (like hemagglutinin in influenza). These changes help the virus escape recognition by the host's immune system, which is why you need a new flu vaccine each year.
• Virulence factors are viral components that enhance the virus's ability to cause disease. These can include proteins that suppress the host immune response or enzymes that damage host tissues.

Economic impact of viral infections

Viral infections carry enormous economic costs across multiple sectors.

Agriculture:

• Crop losses from viral infections reduce both yield and quality. Entire harvests can be destroyed in severe outbreaks.
• Disease management is expensive, requiring pesticides to control insect vectors, development of resistant cultivars, and monitoring programs.
• Trade restrictions and quarantines on infected regions compound the financial damage.

Human health:

• Direct costs include medical treatment, hospitalization, and vaccine development.
• Indirect costs come from lost productivity when people are too sick to work, or from premature death.
• Pandemics like COVID-19 and Ebola outbreaks place massive strain on healthcare systems and disrupt travel, trade, and daily life across entire economies.