The human respiratory tract hosts a diverse community of cocirculating viruses: some of which benignly reside, whereas others aim at acute infections. Interactions between these viruses alter the risk of sickness.
Infecting a cell is a skill. Viruses of the same strain cooperate to be more successful pathogens. Conversely, viruses in different families may compete for the opportunity to inflict illness.
During flu season, influenza viruses sometimes team up with pneumonia bacteria to attack the lungs. Along with the immune system, commensal viruses and bacteria which reside in the respiratory tract do their best to repel the onslaught.
Influenza is a seasonal disease. The adaptive immunity system learns how to best flu virus strains, preventing their ongoing success. So, influenza viruses innovate to overcome learned defenses. Hence, new flu strains arise to vex their victims.
Influenza viruses do not get on well with cold viruses. If attacked by both simultaneously, a potential host has a better chance of not getting too sick from either, as the two pathogens may weaken each other enough for defenses to eliminate the threat. If sick with the flu, the odds of catching a cold are lowered.
References:
Sema Nickbakhsh et al, “Virus–virus interactions impact the population dynamics of influenza and the common cold,” PNAS (16 December 2019).
Astrid A.T.M. Bosch et al, “Viral and bacterial interactions in the upper respiratory tract,” PLOS Pathogens (10 January 2013).
Kirsten M. Kloepfer & James E. Gern, “Ecological and individual data both indicate that influenza inhibits rhinovirus infection,” PNAS (3 March 2020).