Deadly viruses can still be infectious.

Does increased virulence always lead to a decrease in viral fitness? Perhaps not — the virus may still be able to transmit to many others even if its host is very ill, and, if like SARS-CoV 2, the time between infection and death is relatively long, the ultimate death of the host will ‘not matter’ to the virus, since it will have had sufficient time to replicate and transmit to many others in the interim. Australian scientists Anderson and May developed this line of thinking, called the ‘trade-off’ model in the 1970s, which was a direct rebuttal to the previously widely-held ‘law of declining virulence’.

The consequences of the ‘trade off’ model are important: if we follow May and Anderson’s logic, it cannot be assumed that increased virulence decreases the fitness of any given pathogen. Rather, a pathogen’s ‘fitness’ is determined by its individual relationship to the host — increased virulence may or may not, make a difference to viral replication and transmission. No broad-brush statements can be made about the trend of viral virulence over time, we must consider pathogen’s relationship to their host individually.