Biological agents and transmissibility

Some transmission routes are more likely to infect than others.

In general, three macro-categories can be identified: direct transmission, indirect transmission and vector transmission.

DIRECT TRANSMISSION

Transmission by ingestion

This may be accidental ingestion through aerosols or splashes for example.

Per droplet si intende il passaggio di goccioline di grandi dimensioni generate dalla persona infetta (generalmente tramite starnuti, tosse oppure parlando): se espulse a breve distanza possono raggiungere le mucose del soggetto suscettibile.

Contact transmission

It may occur in the case of:

• skin contact (physical transfer of micro-organisms from an infected, or colonised, person to a susceptible person), especially if the skin has open wounds.
• Contact means transmission via the haematogenous route, i.e. contact with the blood of infected animals or humans: it becomes a relevant route in hospitals, veterinary clinics, animal husbandry and livestock farms.

INDIRECT TRANSMISSION

Carried transmission

This refers to the passage through common vehicles and concerns those biological agents that can be transmitted by other materials such as water, food, or drugs.

Airborne transmission

This method of transmission can occur as a result of two circumstances:

• when 'aerosols' are dispersed in the air;
• when the infectious agent is on the surface of fine, inhalable dust.

In the case of so-called 'aerosols', for example, the biological agent manages to remain in the air for a longer period of time, thus increasing the likelihood of contagion.

By definition, aerosols are small liquid or solid particles (less than 5μm in diameter) suspended within a certain gas, such as the air we breathe.

How do aerosols form?

These very fine volatile particles are formed by the mechanical dispersion of a substance.

Some typical examples of aerosols are clouds and fog, in which very fine water droplets are suspended, or atmospheric particulate matter, consisting of a mix of solid microparticles of organic pollutants (carbon residues, metals, fibres, etc.).

In the industrial sector, numerous products are used in aerosol form, such as insecticides, paints, hairsprays, furniture waxes, deodorants, etc., which are emitted in dispersed form through the action of liquefied or compressed gases.

Just consider particle dispersion from spray cans.

Transmission by droplet

This refers to the passage of large droplets generated by the infected person (usually through sneezing, coughing or talking): if expelled a short distance away, they can reach the mucous membranes of the susceptible person.

TRANSMISSION BY VECTORS

Occurs when animals or insects (mosquitoes, flies, ticks, mice, etc.) help carry and transmit the infection.

Stay informed, stay safe.