A method of analyzing biological data, the biological data containing expression values of a plurality of polypeptides in the blood of a subject, each of said plurality of polypeptides being selected from the group CRP, IP-10, TRAIL, IL1ra, PCT and SAA, the method comprising: operating a hardware processor for executing computer program instructions stored on a computer-readable medium, said computer program instructions comprising: computer program instructions for calculating a distance d between a segment S ROI of a curved surface S and a plane π defined by a first direction and a second direction, said distance d being calculated, perpendicularly to said plane π, between a point P on said plane π and a corresponding point on said surface S, said points being defined by a first coordinate δ 0 along said first direction and a second coordinate δ 1 along said second direction; and computer program instructions for correlating said distance to the presence of, absence of, or likelihood that the subject has, a viral infection, wherein when said first distance d is above a predetermined threshold ω V, said computer program instructions determine that it is likely that the subject has a viral infection; wherein each of said coordinates is defined by a different combination of said expression values, wherein said coordinate δ 0 is defined as δ 0 = a 0 + a 1 D 1 + a 2 D 2 +... + φ 0, wherein said coordinate δ 1 is defined as δ 1 = b 0 + b 1 D 1 + b 2 D 2 +... + φ 1, where a 0, a 1,... and b 0, b 1,... are constant and predetermined coefficients, where each of D 1, D 2,... is an expression value of one of the polypeptides, and where each of φ 0 and φ 1 is either zero or nonlinear with respect to at least one of the expression values; wherein said segment is defined between a minimal first coordinate δ 0,MIN and a maximal first coordinate δ 0,MAX, and between a minimal second coordinate δ 1,MIN and a maximal second coordinate δ 1,MAX, said minimal and maximal coordinates corresponding to physiologically possible ranges of the expression values of the polypeptides; and wherein at least 90% of said segment is between a lower bound surface g(δ 0, δ 1)-ε 0 and an upper bound surface g(δ 0,δ 1)+ε 1, wherein said g(δ 0,δ 1) equals exp(δ 0)/(1+exp(δ 0)+exp(δ 1)), and wherein each of said ε 0 and said ε 1 is less than 0.5. computer program instructions for calculating a distance d between a segment S ROI of a curved surface S and a plane π defined by a first direction and a second direction, said distance d being calculated, perpendicularly to said plane π, between a point P on said plane π and a corresponding point on said surface S, said points being defined by a first coordinate δ 0 along said first direction and a second coordinate δ 1 along said second direction; and computer program instructions for correlating said distance to the presence of, absence of, or likelihood that the subject has, a viral infection, wherein when said first distance d is above a predetermined threshold ω V, said computer program instructions determine that it is likely that the subject has a viral infection; wherein each of said coordinates is defined by a different combination of said expression values, wherein said coordinate δ 0 is defined as δ 0 = a 0 + a 1 D 1 + a 2 D 2 +... + φ 0, wherein said coordinate δ 1 is defined as δ 1 = b 0 + b 1 D 1 + b 2 D 2 +... + φ 1, where a 0, a 1,... and b 0, b 1,... are constant and predetermined coefficients, where each of D 1, D 2,... is an expression value of one of the polypeptides, and where each of φ 0 and φ 1 is either zero or nonlinear with respect to at least one of the expression values; wherein said segment is defined between a minimal first coordinate δ 0,MIN and a maximal first coordinate δ 0,MAX, and between a minimal second coordinate δ 1,MIN and a maximal second coordinate δ 1,MAX, said minimal and maximal coordinates corresponding to physiologically possible ranges of the expression values of the polypeptides; and wherein at least 90% of said segment is between a lower bound surface g(δ 0, δ 1)-ε 0 and an upper bound surface g(δ 0,δ 1)+ε 1, wherein said g(δ 0,δ 1) equals exp(δ 0)/(1+exp(δ 0)+exp(δ 1)), and wherein each of said ε 0 and said ε 1 is less than 0.5.