  How important is resource depletion (e.g. depletion of target cells) in
controlling infection within a host? And how can we distinguish between
resource depletion and other mechanisms that may contribute to decline of
pathogen load or lead to pathogen clearance? In this paper we examine data from
a previously published experiment. In this experiment, mice were infected with
influenza virus carrying a green fluorescent protein reporter gene, and the
proportion of lung epithelial cells that were influenza infected was measured
as a function of time. Three inoculum dose groups - 10^4 PFU, 10^6 PFU and 10^7
PFU - were used. The proportion of cells infected was estimated to be about 21
(95% confidence interval: 14-32) fold higher in the highest dose group than in
the lowest dose group with the middle dose group in between. We show that this
pattern is highly inconsistent with a model where target cell depletion is the
principal means of controlling infection, and we argue that such a pattern
constitutes a reasonable criterion for rejecting many resource depletion
models. A model with an innate interferon response that renders susceptible
cells resistant fits the data reasonably well. This model suggests that target
cell depletion is only a minor factor in controlling natural influenza
infection.
