the type of cell it infects. This virus-host interaction will also determine the severity

of an infection. The same virus could have different consequences depending on the

cell type it infects. Despite belonging to different families, viruses have a few

mandatory requirements when inside a host cell:

• Ensure their genome replication: The virus has limited genes and lacks

enzymes needed for genome replication and transcription.

•

Viral protein translation and packaging: As the virus is devoid of ribo-

somes for mRNA formation, they need to alter the host cellular machinery

for their purpose. Some viruses also need to derive part of their envelope

from the host cell plasma membrane.

• Evasion of host immune response: This is a very crucial step not only for

the survival of the virus, but also for release of infectious viral particles.

2.4.1

TYPES OF VIRUS-HOST INTERACTIONS

1. Persistent and productive: Majorly seen in the case of RNA viruses. This

type of infection does not show severe cytopathic effects but alters some of

the host cellular mechanisms such as, proliferation of cell membranes or

endoplasmic reticulum membranes, formation of inclusion bodies. These

viruses do not kill the cell but would replicate inside the infected cell,

resulting in production of infectious particles.

2. Persistent and non-productive: This type of interaction does not kill the

host cell but is also unable to produce infectious virions. They could only

be able to induce the virus.

3. Cytocidal: Severe cytopathic effects observed in the host cells, such as

morphological changes, cell membrane ruffling, and cell death. The cyto-

cidal mechanism could be via alteration in host cell transcription or trans-

lation, inhibition of host mRNA processing or serious change in host cell

membrane. This type of interaction results in the production of infectious

virions that are ultimately released in the environment after host cell death.

4. Transformation: This type of interaction also leads to changes in host cell

morphology. These viruses could pass indefinitely within the host cell

creating a niche for tumor development and could be termed oncogenic

viruses. The oncogenic viruses with DNA genomes would not be able to

form infectious virions, but oncogenic retroviruses would be able to pro-

duce progeny virions.

2.4.2

COMPONENTS OF A HOST CELL INTERACTING WITH VIRAL ELEMENTS

A virus needs to use the host cellular machinery for more than one purpose to

survive and be persistent. To accomplish this, it would need to interact with the host

cell components at various levels, such as:

1. Cytoplasm: Most of the viruses need to gain access to host cytoplasm for

either transcribing their genome in the host nucleus or while viral assembly

Introduction to basic virology

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